First, install CSS content. Idk how gmod hosts work, but if you have to do things via command line and ftp, install css normally and then copy cstrike folder from css folder to orange box folder.
After that, download TTT v28: http://ttt.badking.net/releases/troubleinterroristtownv28
and extract it to orangebox/garrysmod/gamemodes. It should replace old terrortown folder and it's contents.
After that, adding "sv_defaultgamemode terrortown" (w/o quotes) to autoexec.cfg (in cfg folder) should force server to run terrortown gamemode. This should be enough to get the server run latest TTT. If you run into any problems, search these forums (or facepunch) before asking, because most questions already have many answers.