I just downloaded the app but I don't know what my server address is

In order to use the app, you must first sign up for an account at https://dattodrive.com.  You will need to have a sign up code in order to successfully register for an account.

If you do have a code, your server address will be the full string as shown in the "Subdomain" field of the registration form.  For example, if you created an account demo.dattodrive.com, then that will be your server address.