I don't have the instruction booklet of the enlarger, but the carriers should be self explanatory.

I have a variable size carrier. It holds either inserts for various formats or glass plates to make it a universal glass carrier.
To remove the 35mm insert, you need to slide the aluminium ruler inside the carrier which hold the insert. It slides along the two pins holding it, give it a little push if it is stuck, but that's the way to do it.

To use other formats in the carrier you will need either additional inserts or the glass plates. RK photographic can supply you with the accessories that you would need.

Little note, you may have noticed it or not, but you can insert the carrier from the front of the enlarger or the side. It is a very useful feature, to project the image portrait or landscape.

I hope this helps