Alright, for those who are getting problems with the console window appearing and disappearing, it's not the app's fault. Works just fine.
As for those who (like me) have a LOT of .adm files to open and who don't want to open them one by one, I decided to tackle the problem myself.
I tried to make heads or tails of the C++ code, but short variable names confused me, so I decided to take the easy way out instead: The good old batch file.
Create a new text file and rename it <whatever>.bat. Copy-paste this code into it:
for %%n in (dir *.adm) DO "%appdata%\runic games\Torchlight\converter\tldat.exe" %%n
Put it in the same directory as the .adm files you want to convert. The .dat files will be in the same directory (tested).
I've only tried this out on Windows 7. Vista should work, dunno about xp (I don't think %appdata% works in xp), in which case just replace the pathname to wherever you've put the converter.
Oh yeah, this assumes you put the converter in %appdata%\runic games\Torchlight\converter\, for those who can't figure it out. Just edit it if you don't feel like moving that incredibly heavy 132 kb exe file.....