what did you install (MS2 / a snapshot / or what)?
how did you install it? 
kinda sounds like a file didn't get copied or a config error in \catalog\includes\configure.php. .  As to your 2nd problem, chmod 644 should do the trick (after you make your config changes)
there's a good how-to at: 
www.acenet-tech.org/cdj/   - look for the OSCommerce how-to.  
Additional help can be found at 
http://wiki.oscommerce.com/helpDocumentsIf your just starting up a shop, take a look at the secv2.2 contrib from the webmakerscorner.com.  It has a lot of features that most people will probably use and the download includes a copy of MS2 that already includes the changes.  It is a bit different from MS2, so there's an additional learning curve...