If the awning looks like this - it might be a Milliken or at least the same size extrusion:
I had a hard time finding a staple gun that worked, and this is why:
http://www.millikendistribution.com/ae_awning_extrusion_tools.php
"STAPLE GUN #MI11A
The staple gun was designed especially for Milliken Wholesale Distribution. Lightweight, powerful, pneumatic staple guns make the staple pierce the aluminum and the fabric cleanly and easily to hold The fabric securely in place."
I ended up modifying the tip of a Bostitch pneumatic crown stapler very similar to this one:
Bostitch Stapler
(I had purchased it at Lowes as part of a "kit", I believe the stapler is now discontinued) - I removed the safety on it, and ground down the tip to make it to fit inside the channel.
I ended up using fas'ners unlimited 18g 12mm x 6mm staples that were made for another stapler, but they worked out fine.
I had also purchased a Craftman which I couldn't get to work.
Beware of your pressure, I had a few staples go right through the fabric. Good luck with it!
Gene