First of all, may I have the link for the liberty ROM, preferrably the most up-to-date version?
Secondly, as I recall ROM manager does not work with the DX2. I'm sure scur will be on here later and confirm that. If you follow the developer's instructions word for word you won't run into any problems.
1) be on stock 2.3.4
2) install clockworkmod
3) create a backup
4) confirm that the backup was created successfully
5) boot into cwm
6) wipe data/cache (both wipe options)
7) apply sd card zip
8) install the zip
9) reboot your phone and if all goes well you'll be running the liberty mod.
I've been wanting to try the liberty ROM myself;however, I can't find a link with the most up-to-date version. I'd like to try it so compared it to the molten v2.3 rom. After reading the instruction on the CM7 rom, I decided I'm now that knowledgeable yet, so I'll stick with simple flashing roms.