In order for the HSDroid client to access Homeseer, you must have the Homeseer webserver running and available. You must also have a non-guest user account set up.
To configure the Homeseer webserver, go to the Homeseer Setup page, select the webserver tab and make sure it's enabled. Then go to web users tab and create the account you wish to use. This is the account and password you will use on the HSDroid Server Settings page.
Note: If you can't connect from your phone's browser to the Homeseer server, try changing the port from the default of 80 to 81 (this is done in the Webserver page). Then in HSDroid in the Server Settings, change the port there as well.