I have had a couple of people having problems getting the boards to connect and we have found that there can be a couple of things that people skip over in the manual or just do not understand. I will add any help I can to this thread.

1. Are you using the correct Arduino software?
The new versions of the Arduino software have some changes in them that are stopping the board form connecting. Please only use the version stated in the manual because of this.

2. Are you using the correct .ino for your board.
The plugin edits the .ino for each board before it is down loaded and each one of your boards should be different so please ensure you set up then download the correct sketch for the board number you are using.

3. Did you change the board from USB to Ethernet or Ethernet to USB?
If you change the board Ethernet setup in any way you will have to download the board sketch again. This is also true if you change the connection method of the board. This is the only time you need to download the sketch as the rest of the setup is sent form Homeseer to the board.

4. If using Ethernet can you ping the Board.
If you can not ping the board from your server then the plugin will be unable to connect to the board. Please check all firewalls and antivirus that you have running.

If you are still not having any luck there is a step by step guide to getting started Here.