The address of the location is pretty simple. I would suggest that you sue APT or RM for your address. The sample I would note below will use RM since it it more accruate to your current circumstances. Still, either option will be acceptable, and will be delivered. You could probably go completely different, and note Box. as your unit number. Assuming for John Doe living in unit 201, his address will look like:
John Doe
RM: 201
1100 E. Carson St
Pittsburgh PA 15203
If you are like me, and prefer to keep your residence address private, you can take either route to go deeper into South Side to go to the USPS Post Office, or a UPS Store – either one of them supporting offering mail address services.
Also, if you send out a lot of mail, and you write to the same people pretty frequently, I will consider using your word processor to create an envelope for everyone you write to. With that said, yo would want to put your return address in the top left corner, and save that template for when you send a letter.