It looks like it is hard coded to look for the game data directory at C:\Program Files (x86)\Telepath Tactics\Data. Steam puts the data directory into C:\Program Files (x86)\Steam\SteamApps\common\Telepath Tactics\Data. The workaround is to create a Telepath Tactics directory in Program Files (x86) and copy the game Data directory into there.
Actually, it's hard-coded to look for the Telepath Tactics folder in the same directory as the Telepath Tactics Map Editor folder. It'll work just as well if you move the Telepath Tactics Map Editor folder to C: > Program Files > Steam > SteamApps > common--better, in fact, since any changes you make in the actual game's Data folder will then be immediately accessible to the editor.
Sounds good, thanks :)