From what I found out, the Room Finder will only work if you use the dedicated "Select Rooms" dialog (available via the buttons "Rooms..." or "Add Rooms...") and directly enter rooms there. When you specify the rooms in other ways, e.g. in the To field or the All Attendees list, Room Finder will consider the rooms to be regular attendees, and e.g. not show them as "available rooms" in the Suggested Times list.
So a straightforward way to use the Room Finder is to manually select multiple rooms in the "Select Rooms" dialog.
This however may be inconvenient if you have a long list of candidate rooms. To solve this problem, you can make use of a local Contact Group. Create such a group, name it "My Rooms", and add the rooms to it that you want to consider for meetings. Then, create a new Meeting and use the group in the following way:
- Enter the group name "My Rooms" in the To field
- If Outlook doesn't recognize the group yet, click on "Check Names"
- Click on the plus sign to the left of "My Rooms" and confirm the dialog
- Cut all the rooms in the To field (CTRL+A, CTRL+X)
- Enter the attendees
- Open the "Select Rooms" dialog (ALT+M or click on the Rooms button to the right of the locations text box)
- Paste the rooms in the Rooms field and confirm the dialog (CTRL+V, ENTER)
Now the Room Finder show will show available rooms from your personal list of candidate rooms.