The problem was that it wasn't installed properly,so that the play button didn't show up
Now it works just fine, for some strange reason

Btw thanks for your help!

What do I have to write on the command prompt to run the map?
The mod file name is "Snowy Town" which is unusual because usually the mods names are named with an additional "nazi_zombie" part at the beginning of the modname.
I really don't know how to run this.