Importing the MIB is not really required if you just want to change it from "Unknown Device"
Go to System -> Resource Management -> Device Model. Click "Add". Fill in the model name. Easiest way to find the sysOID is to query the existing device. Set the Vendor, Series and Category as required. Fill in the description field if you like.
The device will then be updated to show the new model name. That's all you really need to do. You don't HAVE to import any MIBS for regular management. However, if you want to import the MIB to use it for things like the mib browser, or for traps, there's two places to do that:
For trap definitions, copy the MIB to the IMC server, under <imc>/client/TrapMIB. Then go to Alarm -> Trap Management -> Trap Definition. Click "Import trap definition from MIB file" and follow the instructions.
Note that the HP MIBs are already imported, and I would be surprised if there's much in the way of new traps for this specific device.
For use in the MIB Browser, go to System -> Resource Management -> MIB Management. Go Tools -> Load MIB Files. Click the icon that looks like a screwdriver and a crescent. Follow the instructions to load & Compile the MIB.