I had trouble discovering servers on a different subnet so after a little digging, I found the below which did the trick.

http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=2014366

 

After installing the VMware Capacity Planner Datamanager and configuring the system to receive new systems, you can perform these steps to discover systems on an IP subnet:

1. Open Capacity Planner Data Manager.
2. Navigate to Admin > Options.
3. Go to Modules > Discover > Settings.
4. Deselect the Discover Groups / Domains option.
5. Under the Node Discovery tab, deselect Lan Manager Systems and Active Directory Systems.
6. Select DNS Names and IP Address.
7. Under the System Discovery tab, select the Test Connection to system during discovery option.
8. Under the IP Subnets tab, click Add.
9. Add the appropriate Subnet Range by Class or CIDR Notation.
10. Click Apply.
11. Click Tasks > Run Manual Tasks > Run Discover Task.The IP subnet that has been configured are pinged and systems are discovered.