Installation failed, cannot edit php code

Hello,
I tried to install WordPress 3.0, but got the following errors. The first erros was without deactivating the plug-ins:

Could not create directory.: /public_html Installation failed

And this error was with deactivating all the plug-ins:

“this file couldn’t be replaced”: /public_html/wp-admin/css/wp-admin.dev.css

It seems that 3.0 was partially upgraded and now I cannot edit the php code. Also, do I need to change the permissions of my directories before I upgrade the software?

I was going to try manually upgrading, but the files dates look to be updated already.(?)

Thank you in advance.