Once the resources for a given unit have been setup in Nexus and the access permissions set by the Resource Manager, users with permission to book resources will be able to book those resources using their chosen calendaring application. For Mac users on Lion this means that you can book resources, or rooms from iCal in the same way that you already use iCal to set up meetings.
Booking a room using iCal is actually very straightforward because you simply treat the room as just another invitee to your meeting. All of the available resources (these are usually rooms) appear in the Nexus Global Address List in exactly the same way as if it were a person. So, for example if you wanted to book an OUCS meeting room, say the OUCS Babbage room, you would simply add a new event to your calendar and you would "invite" the OUCS Babbage room to that event by selecting the room from the address book in exactly the same way that you invite other Nexus users to your event. The room will then appear in the list of invitees and you can check availability in exactly the same way as you do for your other invitees. Once you have sent the invitation you will receive an automatically generated meeting acceptance or rejection message from the room according to whether it is available or not. If you wish to book a resource where it doesn't make sense to have any other invitees, for example, your unit may use Nexus to manage resources such as a projector, you will effectively be setting up a calendar event with just one invitee, the resource in question.