As Tom points out, your Global Entry application is only completed after the interview. The Conditional Approval notice is only enough to schedule an interview, not to use GE.
At the end of the interview, the CBP officer will tell you if your application was successful or not. If it wasn't, they may say they need to check further things (I've read about one person on FlyerTalk who had to wait another week).
Assuming that you passed the interview, and they welcome you to the program, then about an hour after the interview you can log into the GOES website and view your Global Entry Membership Number. That number can then immediately be used as your Known Traveller number for TSA Pre. Later that day, you'll also get an email from
An hour or so after the interview, you should be able to get your number from the GOES website. Later that day you should get an email from CBP with a subject like "GOES - Account Status Change". That won't have your membership number in, but is a sign that when you log into GOES you'll be able to download the formal membership approval letter which does have it. (Or you can just see the number once you're logged in!)
If you're a US permanent resident, they'll also post you a GE card, which may take a few weeks to arrive. However, your membership is active once your successful interview is over, and you can use GE terminals + TSA Pre from then on even while waiting for the physical card. (Non US residents don't get the card)