I had recently placed the Showtime performance venues in Bridgeport. Then, I created a sim and used the Nraas MasterController mod to give them a job in the singing career at the top level. I wanted to audition him for a gig, but I couldn't because none of the venues had a proprietor. How do I give them a proprietor? I have already tried having him purchase the venue and it doesn't work.

U Know how u got that town with the showtime , Thats where proprietor's are . U Wouldn't really find them in bridgeport , sunset valley, Twinbrook etc... just use the town that came with showtime

I don't have Showtime. I'm guessing.

With the other EPs you had new buildings added for the new functions. (Although I was a bit amused by it adding the junkyard into a vacant lot next to City Hall in a custom world when I got Ambitions...) If it didn't add anything, maybe you need to sell and rebuy the theatre, stadium, etc. and use the versions of the rabbitholes that came with Showtime? And be aware that the new building might not qualify as the right one for the photography collections.

When you build a new showtime lot, it takes a while for the game to generate a proprietor placement point.

You should play your game for a few sims days to see if the game generates the placement point. If it doesn't, you could download an override which allows the Proprietor placement to be buyable. I have one here:
http://modthesims.info/showthread.php?t=427515
(Post #3)

Thank You Simsample, Thanks so much for everything you do for us all

Actually, if you set up a venue and wait a while and then view the town you can see the proprietors walking to the venue. Wait until they go inside or are standing on the lot, then you can audition. It worked for me and I put them in Midnight Hollow.

Alternative way:
Ctrl+shift+C testingcheatsenabled true
Ctrl+Shift+C BuyDebug

*If you don't want to create a lot from scratch*
Shift+click on the ground on the lot you want to create the stage on
click "buy on this lot" or "build on this lot"

Click the ? box:
1. Misc. tab (far right)
2. Set filter to just Showtime objects (Star icon)
3. Create stage and audience using markers and objects (read object descriptions)
4. Save and exit the lot.
5. Wait for proprietor to arrive on the lot.
6. Audition!