Do we need to wait until September to walk into Gilmour Hall and get our letter of enrolment?
Also do we need to fill out the form that is online when we go to get the letter?

When I need to get money out of an RESP I usually just print off my Program and Course Summary from MUGSI. Haven't had any problems before.

(Mugsi > My Mugsi > Academics > Program & Course Summary)

I'm pretty sure you can go any time in the summer, you don't have to wait until September. Just check the summer hours on their site because they close earlier.

Make sure you've confirmed your payment agreement and you should be good to go now - that's all I needed as far as I remember.

As for the online form I'm not sure - my RESP provider gave me a proof of enrollment form to give to the registrar to fill out rather than having the registrar provide its own letter.

Your letter of acceptance and your payment agreement should suffice, at least it did when I used it a few years back. Having the program summary might help though. You could also go and get a letter from the registrar now if they need it (maybe you can also for it to be mailed/faxed instead of going in person)