How do I add custom models to my map? Could I add an image such as a .png as a zombie model? Sorry for the stupid question 

You need Maya and a plugin to export things. Maya is a program for 3D modelling. You can download 3D models from the Internet, open them with Maya and export them to cod. Or create yours. It also works with anims.

You can download a free student version of Maya in http://www.autodesk.com/education/free-software/maya.
And the plugin here http://ugx-mods.com/forum/index.php?topic=1295.0.

You can also use Blender but I don't know how you export things with it.

Once you've exported the model you'll have a "xmodel_export" file (I think it's the name). You'll have to convert the file to a model with Asset Manager. Then you can place it in your map. I think you also need to add this line xmodel,NAME_OF_MODEL in your mod.csv, but not sure if it's necessary.

If you only want to change zombie's skin you can look for their .iwi image and convert it to .dds with Tom's IWI to DDS tool, edit them in PhotoShop, Gimp, Paint.net... Save the image again to .dds (you'll need another plugin for this) and convert it to iwi again with Asset Manager (without overwriting the original file, so give it another name). Place the new iwi in your mapname/images folder and give it the original name.

Hope it helps.

I think you also need to add this line xmodel,NAME_OF_MODEL in your mod.csv, but not sure if it's necessary.

I for one has never had to do this, except for viewmodel & worldmodel for guns. Anything made in radiant, I have not had to include into mod.csv

You need Maya and a plugin to export things. Maya is a program for 3D modelling. You can download 3D models from the Internet, open them with Maya and export them to cod. Or create yours. It also works with anims.

You can download a free student version of Maya in http://www.autodesk.com/education/free-software/maya.
And the plugin here http://ugx-mods.com/forum/index.php?topic=1295.0.

You can also use Blender but I don't know how you export things with it.

Once you've exported the model you'll have a "xmodel_export" file (I think it's the name). You'll have to convert the file to a model with Asset Manager. Then you can place it in your map. I think you also need to add this line xmodel,NAME_OF_MODEL in your mod.csv, but not sure if it's necessary.

If you only want to change zombie's skin you can look for their .iwi image and convert it to .dds with Tom's IWI to DDS tool, edit them in PhotoShop, Gimp, Paint.net... Save the image again to .dds (you'll need another plugin for this) and convert it to iwi again with Asset Manager (without overwriting the original file, so give it another name). Place the new iwi in your mapname/images folder and give it the original name.

Hope it helps.

if the model is placed in the map it doesn't need to go in mod.csv

Thanks for the help guys! I am trying to make a minions map with minion models  (not from Die Rise, from the movie)

Also, do you know if there is a way to add the more gore mod from Requiem? Cause I would think it would be necessary when slaying minions 

Also, do you know if there is a way to add the more gore mod from Requiem?

Yes it's called UGX mod, maybe you've heard of it?