There are several reasons why the Cruise Control does not work, with the lever (brake and clutch) microwitches being high on the list, specially if aftermarket levers have been fitted. Have a look at this thread and also do a search with the word "cruise" .
I had the same problem with mine a while back, all I did was give all the micro switches a good blast of WD40, worked all the levers and pedal, more WD40 more working levers etc and then it was fine again and has been ever since.
i would imagine that its your front brake lever, listen very carefuly to hear if there is 2 small clicks as you opperate the lever? I have seen this problem so many times especially with after market levers. you must first turn on the cruise control which brings on the cruise light then press and hold the set button. what happens is the milling where the micro swich sits can wear or incorrectly milled and will not be holding the micro switch outward instead holding it in telling the cruise that you are applying the brakes so disingage.
hope this helps?