Hey, there. The default IV:MP font is called Tahoma Bold. But, you could also use Tahoma. It's not such a big difference.
When you are going to your settings in IV:MP, go to "Chat" and type the following text inside the "Chatfont" field (the first field), which can be found below (if you want to stick to the default font, just as Dino said)
tahoma-bold
use "tahoma-bold" 
Anyway, make sure the font is there in the "fonts" folder inside the "CEGUI" folder, which can be found inside the "IVMP" folder. Hope I helped!
