Logon locally to the servers console or access the resource through the network?
ISA Management permissions are in the ISA manager. Right click the server and do properties (same place where you can find outgoing web requests, incoming web requests, auto discovery, etc.)
Add whatever you need to the security tab. That may do it. If not, right click on whatever else you find that may apply to your needs but don't overdo it and change things you don't know about. I recommend doing a backup of the ISA config first....