What or how do i have to convert the mshes to be able to get them into Mod Tool ? I used search, but I couldn't find anything useful. (I'm still a n00b at modeling in XSI, but I'm getting there )

You can use Meshtool to convert them to obj files that can be loaded directly into XSI (albeit with some possibly unwanted things - Meshtool converts the shadowvolumes and collisions as well). Or you can use MSHViewer to convert them to vrml files, and another program (such as Vivaty) to convert those to .xsi files, which you can then load in XSI. This second method is a bit more involved but has much cleaner results, so I'd recommend it if you're looking to convert any vehicles (which have a lot of collision geometry that can be messy after conversion with Meshtool).

Ok thanks I'll try both, I actually just want to load weapons into XSI and modefy them.

forums/viewtopic.php?p=431929#p431929

Read through the whole thread for various conversion options.