Yep, the address on the envelope is fine
it has the office name (office of the registrar) and building location (GH 108).
You can put your return address in the upper left corner of the envelope, in this format:
First name Last name
123 Fake St
City, Province
Postal Code
If you're mailing from a country other than Canada, you should probably write the country name in underneath the city/province.
The stamp goes in the upper right corner. They might have given you an envelope with postage paid already--so if there's already something in the upper right corner, you're probably fine
If not, you need a stamp there.