Hi,
I'm having trouble with adding a custom spell ingame by commands,
can't seem to figure out why.

I'll give you a look of how the spell looks in the Creation Kit:

http://i91.photobucket.com/albums/k284/malmern/CreationKit_zps452ecde9.png

In-game, open the console and type

help "tri-force"

That will give you the FormID of the spell to use. It'll be something like XX003320, where XX is the variable prefix of your mod. It will NOT be 02. Then you should be able to do:

player.addspell XX003320

to actually add the spell to your Magic/Powers menu.

Thanks for the fast reply!

How could I forget this simple fact,
I've done this a dozen times with items already...

Happens to us all. Glad to help!