TA_UseTrial() does not return true or false. It returns an HRESULT with TA_OK, TA_FAIL, or one of about 30 other error codes. The error codes are important and tell you exactly what is happening.
So:
1. Make sure you're using the latest version of TurboActivate.
and
2. Give us the exact error code you're getting (or, save yourself some time and look up the exact error code in TurboActivate.h and see a full explanation for what it means).