How can I change the Mode in a room? When I open a Room it is always akido, but i want another mode like Jooust or something.

Press ctrl + m then select your desired mod.

Type in
/set mod ModName.tbm
For example
/set mod taekkyon.tbm
But be careful, they are Case sensitive....

but i need the mod on my computer? i thought i could just write for example

/judo or /joust

Yes. The mod must be located in the Mod folder in the Toribash DIR. If you don't have it, it won't load. Not all mods load by the way. I think this is due to the other person does not have the desired mod.

You can press esc, then click mods, the click the mod you want.
Or you can just type /set mod <mod>.tbm

There you go