Then in the second field put a link in to:
a) If your extension is just a free direct download link - then put in a link to the update XML you have or direct to the zip file
b) If your extension requires registration or is a commercial extension then you need to put a link to your site in (generally your sites registration page).
If your extension is a free direct download link then all your work is done and you can ignore the next step :)
If you require a consumer to register or be using a commercial extension then you have to set up a plugin on your site to deal with the user registration. A good starting point is a plugin that was developed specifically for this purpose found here.
This plugin is specifically designed for the Joomla User Component however it will also integrate nicely with Community Builder simply by changing the entry point URL in the plugins parameters - generally it is recommended (but my no means compulsory) to point this to your login page. Note an empty value of this parameter will default to pointing to the Joomla User Component Login page.
The final parameter you have to set up is to point the plugin to your extensions update XML file or zip file. This is done in the Files to Download parameter
On each line put the JED number of your extension followed by the link to the extension for example:
This will need more work than the previous step. It is advised that you download the same plugin - however you will have to integrate into the plugin an extra check to see if the user has purchased your extension. As different extensions implement this in many different ways it is left to extension developers to implement this functionality.