Macybaby, Congrats on owning a Two Spools! It's my favorite machine for piecing since it has the perfect 1/4 foot and the feed dogs teeth are both the same length to feed the fabric evenly through.
Is that manual for the Two Spools or the attachments? If you have the Two Spools manual, there are directions on adjusting the Spool case spring plunger, which is what may be preventing you from getting the spool case to clip in place. If you don't have these directions, I will be happy to post them here. You will need a very small, thin screw driver head to make the adjustment.