Gravity Forms installs just like any other plugin for WordPress. You can find installation instructions in the documentation section of this site at URL listed below.
http://www.gravityhelp.com/documentation/installing-gravity-forms/
If you've already activated Gravity Forms, you can click on the Forms link in the left sidebar, the under forms, click on the settings link.
At the top of the settings page, you'll see a field where you can enter the license key that was included in the email when you made your purchase.

After you've entered your key, click on the "save settings" button at the bottom of the page.
This error typically happens because of a PHP version conflict.
Parse error: syntax error, unexpected T_STRING, expecting T_OLD_FUNCTION or T_FUNCTION or T_VAR or '}' in /.../../../wp-content/plugins/gravityforms/gravityforms.php on line..
Gravity Forms requires PHP5+ and judging from this error you are trying to activate the plugin on a site running PHP4. Check with your web host to find out how to enable PHP5.