Not sure if this was addressed before and I'm somehow using an old version of the mod, but I found that upon selecting the "1453 - New Age" campaign, the game would CTD while loading. A quick look at system.log.txt revealed the problem, and lucky for me, the solution was as simple as editing a text file.
The Problem:
There are two lines regarding Prussian Axemen that the game can't parse, because of a missing space after a closing bracket.
The Solution:
1 - Open "Medieval II Total War\mods\conquest\data\new_age\export_descr_buildings.txt" in a text editor that provides numbered lines.
(Notepad++ is great for this sort of thing, and I've found it absolutely invaluable for modding games, and troubleshooting mods that both myself and others have created.)
2 - Navigate to line 7872, which should read as below.
Code:
recruit_pool "Prussian Axemen" 1 0.34 2 0 requires factions { teu, }and hidden_resource baltic and hidden_resource west and hidden_resource charm
Change it to this instead.
Code:
recruit_pool "Prussian Axemen" 1 0.34 2 0 requires factions { teu, } and hidden_resource baltic and hidden_resource west and hidden_resource charm
3 - Navigate to line 7987, which should read as below.
Code:
recruit_pool "Prussian Axemen" 1 0.34 2 0 requires factions { teu, }and hidden_resource baltic and hidden_resource west and hidden_resource charm
Change it to this instead.
Code:
recruit_pool "Prussian Axemen" 1 0.34 2 0 requires factions { teu, } and hidden_resource baltic and hidden_resource west and hidden_resource charm
4 - Save the file you just edited.
5 - Re-run "Conquest XP.bat" or "Conquest Vista.bat", and select the 1453 campaign again, so it overwrites "export_descr_buildings.txt".
Optional Step 5 (If you're not sure, or prefer not leaving any opportunity for failure open, don't try to do this!) - You could navigate to "Medieval II Total War\mods\conquest\data\export_descr_buildings.txt" and edit the same CTD-causing lines in that, after editing the one I mentioned above, to avoid having to re-run the .bat file, but I prefer to leave nothing to chance. Editing the same file twice leaves room for a slip of the finger, where with the method I listed above, you can be sure that the change you just made will be applied for you.
Hopefully this helps other people enjoy this amazing mod a bit more. Seriously, mad props to uanime5 for Eras Total Conquest. Eras is tied with Mongolphobia for "Most fun I've had in Medieval II."