Advice for a newbie please

Howdy guys.

I am without internet on my PC at the moment and I am attempting to use my ASUS WL-520GU as an access point so that I can temporarily leech off of the outlying networks in my community.

The problem stems from attempting to set it to router usage in administrator settings over its web GUI.

When it asks that I input a static IP i get a promp telling me that my subnet mask is using "an invalid IP address".

The manual says to use 255.255.255.0 but when I try to I get that error prompt.

I am running a Vista desktop.

Thanks in advance for all advice.