NOTE: Room Layouts are stored at the 'Global' level and are not unique to each Event.
STEP 1: Go to Functions > click 'Room Layouts' under the 'Settings' header:
STEP 1: Go to Functions > click 'Room Layouts' under the 'Settings' header:

STEP 2: Click the 'Add New' button:

STEP 3: Enter room layout information, then click 'Save'

NOTE: this screen refers to "Lookup Values" because you can also access these settings under 'Settings' > 'Lists'