Thank you for your post.
One of the most common problems with people running PHP Site Assistant the first time is not setting up the database file properly with the extended privilege. Therefore, go into FileMaker Pro 10 and open the database file. Pull down the File menu and select "Manage -> Accounts & Privileges...". In the next dialog box, click the "Extended Privileges" tab and make sure fmphp is listed. If not, then add it. Then, click the "Accounts" tab and double-click on the account name that you will be accessing with PHP. Click the "Edit..." button to the right of the Privilege Set, and under the Extended Privileges, make sure there is a check mark next to "Access via PHP Web Publishing - FMS only (fmphp)".
Quit out of FileMaker Pro and launch FileMaker Server and start the Admin Console. Make sure under Overview that the Web Server and PHP are turned on. Select "Databases", and make sure the file is set for PHP access. After that, then launch PHP Site Assistant, create a new site, connect and you should then be able to choose the database file.
If you are still running into difficulty, or if you are having difficulty with any of the above steps, please let me know.