You don't "apply zenpacks to servers". The Zenpacks are just a way to add functionality to zenoss. Different Zenpacks provide different functionality, and no single approach is used to fully take advantage of its features. The documentation for the pack should inform you of what needs to be done. I'm making an assumption that docs/DOC-3394 is one of the packs you are referring too? If so on that page it provides you with the following instructions:

Go to the specific device (yes, it needs to already be added).

1. From the > Menu select More -> Collector Plugins

2. Remove the zenoss.snmp.Cpu (click the X next to it)

3. Click "Add Fields"

4. Drag and drop all of the "community.snmp.HP*" entries in the right list over into the left list. Note that community.snmp.HPLogicalDiskMap,  community.snmp.HPHardDiskMap and community.snmp.HPExpansionCardMap plugins must be at the end of lists.

5. CLICK SAVE (don't forget this step)

6. From the > Menu select Manage -> Model Device

7. Once it is completed modeling check out the hardware tab for the device, you should see CPUs, Harddrives, Fans, etc. If you don't you may have problems with snmp, so check those settings the usual way.