hI smoggy,
you lucky thing having a trip to kl. I lived there for nearly 4 years from 2000 to xmas 2003 and another year in 96/97 for work.
its a great place however i did not do any mountain biking whilst there.
i did see a thread some time ago - maybe in the who, where what bike - or users bikes.
there was a club called 'weredefakawi' or someting like that.
in my experience i reckon there would be millions of places to ride in the jungle etc.
be warned the temperatures are crippling at 32 dgrees c as a minimum and 90% humidity - i used to play footie and it was only from 5-7 in the evening as before that time it was too hot - though definately a bit cooler in the shade of the jungle if u end up going to routes like that.
a great idea might be to get to cameron highlands - maybe an hour or two drive from kl and a lot cooler due to altitude - plenty of nature reserve type trips available - though not sure how they deal with mountain bikes. maybe genting resort where they have the casino -since gambling is illegal in malaysia yet they still alow a casino suggests they may not be too bothered if your blasting around on a bike?
in kl itself i am struggling to think of places that may be appropriate for riding - like i say i did not do any biking over there?
general terrain off the roads could be good but jungle should really be avoided as it is too thick and dense - maybe attach a couple of machetes / parangs - to your axles and see what happens

also the wildlife can be a bit mental - mosquitoes, leeches are the worst but then of course you have man eating tigers and pythons - and zillions of poisonous snakes and tarantulas. nice. and if you have an accident dont expect a lot of immediate help from the emergency services. be prepared for the worst and be very well insured.
on that note - prepared nature trails in places like the cameron highlands are probably your best bet.
hope that helps
ian