You posted in an almost 2 year old thread (initially) about Maemo 4. Your Nokia N900 has Maemo 5.

Skype isn't an install on Maemo 5, you just have to go into your account settings and add your credentials there.

Maemo 5 forum is located here and the Nokia N900 forum is located right here.

You will want to check and search those forums for your answers. These things have been answered before.

Good luck!