Well, I'm having the same issue in IMC 7.2 E0403P10.
I've downloaded the 2 MIB files from Fortigate.
Then I go to Alarm->Trap Management->Trap Definition
Import trap definition from MIB file
MIB Upload
Select "Using the System MIB Library" as Yes
Select the MIB file from my download directory and press Upload.
I get a Failure message that says the FORTiNET-CORE-MIB imports from the missing INET-ADDRESS-MIB
Any suggestions on what is going on?