Simply copy an existing facility file, such as flaktower.xml, and rename it to whatever-you-want-it-to-be.xml.
Open it up and you'll find this:
<Facility Type="Flaktower" Flags="isMilitary" OuterDistance="10000">
<Units>
<Unit Type="#flaktower" Position="0.00 0.00" Angle="0.00"/>
</Units>
<GroundPlanes/>
<Routes/>
</Facility>
Change Facility Type="Flaktower" for Facility Type="Opera House" where "Opera House" is the name of the building you want to see.
Unit Type="#flaktower" designates the model name, so the m3d name of your building, without the .m3d tag. So here, you'd put Unit Type="ber_3"
Chuck it back into the facilities folder, and you should find you can find it.
Alternatively, if it exists as a separate facility file in stock CFS3, you should simply copy it over, after making sure the building it refers to is actually in the MAW buildings folder! If it isn't, copy that over, too. It will be, but make sure anyway.