This source file (e.g. senses30.src provided with FreeLing) must contain the word list for each synset, one entry per line.
Each line has format: sense word1 word2 ....
E.g.
00045250-n actuation propulsion
00050652-v assume don get_into put_on wear
Sense codes can be anything (assuming your later processes know what to do with them) provided they are unambiguous (there are not two lines with the same sense code). The files provided in FreeLing contain WordNet 3.0 synset codes.