You don't need to have a maemo garage to upload to extras-devel (least I don't think so... but can't be 100% certain). Try uploading through here

https://garage.maemo.org/extras-assistant/index.php

and I use these instructions to get it ready

http://talk.maemo.org/showthread.php?t=39962

Edit: I was partially right.

Edit 2: I know you can also generate an SSH using the Generate SSH key button if you set up the maemo device configurations (allows running the Qt app on the N900 for testing - works through madde developer).

If thats still not working, or if its the same key, make sure its set up correctly. Mine has the ssh-rsa on one line and then the key on a seperate line. I noticed if I opened up the rsa.pub using notepad it stuck them all on one line. Not sure if it makes a difference, but worth a try.