im wondering how to add .mts or .thm files with mIRC 6.17

Do add MTS themes, you need to be running a script that supports and implements MTS. You should be able to find such scripts on a mIRC scripting site (search mIRC script on google).

-genius_at_work

edit

1st of all; please don't advertise full scripts when only an addon is required. The same addon included with that full script can be found here.
2ndly; this forum doesn't help anybody use mIRC on MSN. This is including but not limited to giving people MSN scripts, pointing people to scripts that are capable of connecting to MSN and telling people how to connect to MSN in mIRC (or any client except the official one, for that matter).

Please edit your post accordingly.