There are several reasons why you might be having problems:
1. Your firewall must allow connections on that port. Be sure to add an exception to your firewall for the PG App on this port.
2. Your anti-virus software might be getting in the way. You may need to configure your AV software to allow the PG App.
3. Your device must be on the same network as your development machine.
4. You don't mention what version of the PG Desktop you are using -- make sure it's the most recent version. Also make sure the associated PhoneGap Developer app on your mobile device is also up-to-date.
5. Review the welcome text for this group, as it contains useful information as to what we need from you in order for us to help you best.