go into webadmin, defaults tab, voting gameconfig, add your mutators to the list there.
When you download a mutator install instructions are usually included mostly you just need to drop some stuff in the system folder of your red orchestra. After uploading the files just restart the server (not the map really restart the server) and you're good to go.
Some mutators need to be added to the serveractors list in the ini (hardly any though as this can be bypassed)