Carry out the following steps to install an add-on:
Go to your dashboard and click on Add Functionality.
Before you can access the marketplace, you have to connect to the concrete5.org community. Click on Connect to Community. You're redirected to a screen where you can enter the details about your site.
Enter the information about your site. No one is going to see this information; it's only supposed to make it easier for you to manage your sites. You also have to create a concrete5.org community account if you haven't done it already.
Note
This has nothing to do with the accounts you've already created within your site. This is an account for concrete5.org. You can use one account to connect all your sites to the marketplace; you can also use it to access the support forums on concrete5.org. If you haven't created an account before, click on Create a new account on concrete5.org and enter all the requested data, as shown in the following screenshot...