The Sims 3: Showtime And (vs) Late Night

Hello all!
This is my first post, so sorry if it's in the wrong section.
Anywho:

I've downloaded The Sims 3, and Showtime.
I've been playing Showtime, but saw you can get an acting career through Late Night (The highest paying career, it looks like).
So, I've now downloaded Late Night.

My question is:
How do I go about installing Late Night?
Can I just install it without uninstalling everything else?
Which exe do I use to run the game? Showtime, Late Night, normal Sims exe?

Thanks in advance for any help!

//EDIT

I should add, I've cracked my game (Obviously)..
Installing Late Night worked, and I'm going to manually patch/ultimate fix my game;

But I need to know what launcher to use to get all of the expansions additions when I'm playing.

Hello,

This is the correct installation method: http://games4theworld.bestgoo.com/t1171-must-read-how-to-install-the-sims-3-correctly

If you don't know which .exe or launcher to use, this should clear up: http://games4theworld.org/sims3faq.html#always_get_in_same_game