Hi, Nipuna,
I think you better go and meet authorized personnel at BIT office, at Dharmapala mw.
Actually you don't need to wait for a letter. you can go and register. Even u can register online at
http://www.bit.lk.
when u r going to BIT office, make sure u take all necessary doc with u. (eg: ur result sheet, ID, etc).
PS: this is to remind u BIT office is closed on 24th Dec
RGDS
Uditha