Googling the words tribe, penis and knot I found:

http://nawtythings.com/penis.html

The Caramoja tribe of northern Uganda tie a weight on the end of
their penises to elongate them - sometimes to such a degree that
the men literally have to knot them up - while the Mambas of New
Hebrides wrap theirs in yards and yards of cloth, making them look
up to 17 inches long.

Googling Caramoja I find 253 pages, most of them repeating the same
story in English and other languages. One page
www.caramoja.churchirrigation.com had the word Caramoja repeated many
times in the text which Google indexed, but viewing the page directly
caused the browser to autoload a totally different erotica/porn site.

This page has more details:

http://www.eviloverlord.co.uk/humour101/humour136.html

The Caramoja tribe of northern Uganda tie a weight on the end of
their penises to elongate them--sometimes to such a degree that the
men literally have to knot them up . . .

From http://www.lasvegasweekly.com/features/your_so_vain.html :

Of course, few actual penises seem capable of living up to their
symbolic attributes, which is why most scholars feel that the more
importance men attach to the penis, the smaller it appears. That may
explain why ritualistic penis enlargement has been a feature of many
cultures. The most popular technique, tying weights to the penis, has
been practiced by groups ranging from the Caramoja tribe of Northern
Uganda to the Sadhus of India. The Sadhus, a cult of ascetics who can
be found along the banks of the Upper Ganges, believe that God dwells
in the penis. By attaching weights to their fishing rods from the
time they are young, they stretch them to lengths that range from
12 to 18 inches.

The page http://www.penilefitness.com/p3.php has photos, but nothing
which seems to be suitable for tying knots.

Google finds 29,000 pages for "Sadus" - 878 of which also have the word
"penis".

http://www.bugbog.com/exotic_places/nepal_travel/nepal_travel_2.html

Sadhus practice severe self-discipline, even self-torture on their
road to total body control, magical powers and nirvana. They start
with celibacy, silence, starvation, and painful body positions. There
are recorded instances of sadhus who never lie down or who never
stand up, who hold one or both arms permanently over their heads or
who stand on one leg.

Some sadhus stretch their penis' from an early age by hanging weights
on them. Over 60cm is not unusual, but the organs no longer function
as sex tools at that length, in case you were thought it was a good
idea.

Some sadhus sew up their mouths and consume liquids only, some have
coconuts or other fruit sewn onto their bodies, or hang from a frame
by hooks through their flesh.

But these extreme cases of self-torture are rarely visible in Nepal.
The sadhus there are more interested in gain than pain. Life for
sadhus in Nepal is too good to be blue.

Firstly they have total freedom - of worship, of travel, of
expression, from social and familial responsibility, from want, from
fear. They are also freed from the chains of their minds by the
constant use of hashish or marijuana, illegal in Nepal but permitted
to Shivaites as part of their religion.

I can find nothing on such penis enlagement techniques or the Caramoja
in the Encyclopedia of Unusual Sexual Practices, by Brenda Love, which I
think is a very well researched reference. There used to be a website
for this book but I can't find it now.

The most obvious conclusion is that this Caramoja story is fictional -
but perhaps there was such a tribe, and perhaps there is a report from
an antropologist in the 1920s to the effect which is hard to find.
Hmmm, maybe it is a misspelling . . .

Googling:

tribe "northern uganda" penis -caramoja

I find 51 pages with similar stories with different spellings for the
tribe. The numbers are how many pages Google finds for this word, which
do not contain "penis", followed by the number with "penis":

http://feastofhateandfear.com/articles/penises.html

Karamojo 1280, 20 (all seemingly rubbish)

http://www.phallosan.com/us/fragen_antworten.html

Karamogong 10, 7

http://www.waikato.ac.nz/wfass/subjects/geography/newsletter/a5.pdf

Karamojong 5170, 930

A more intrepid researcher may be able to use these leads to determine
whether or not this is a myth.

- Robin