Installing PHP on IIS v5.1

I have went through the manual installation instructions exactly, but when I get to the part where it says: If you want to use the CGI binary, do the following: a.. Under 'Home Directory', 'Virtual Directory', or 'Directory', click on the 'Configuration' button, and then enter the App Mappings tab. b.. Click Add, and in the Executable box, type: C:\php\php.exe for PHP 4 or C:\php\php-cgi.exe for PHP 5 (assuming that you have unziped PHP in c:\php\). c.. In the Extension box, type the file name extension you want associated with PHP scripts. Leave 'Method exclusions' blank, and check the 'Script engine' checkbox. You may also like to check the 'check that file exists' box - for a small performance penalty, IIS (or PWS) will check that the script file exists and sort out authentication before firing up PHP. This means that you will get sensible 404 style error messages instead of CGI errors complaining that PHP did not output any data. You must start over from the previous step for each extension you want associated with PHP scripts. .php and .phtml are common, although .php3 may be required for legacy applications. When I got to the App Mappings tab and try to add the path and the extentions my OK button is greyed out, therefore it will not let me add them. Anyone have any ideas?