This method was tried and tested during beta, so let me present it as a working method in the released version too.
All you have to do to make NIS scan files coming through Trillian is to manually enter the path of the IM scanner into the appropriate option in Trillian.
Screenshot from Trillian5.
The appropriate path for the current version of NIS is: C:\Program Files\Norton Internet Security\Engine\18.1.0.37\navwnt.exe
Just remember one thing: when the version number of NIS changes, you will have to update the path you entered (the part in bold numbers).