Hi Hope512, welcome to the PC Simming world! Do you mean the big two-by-two pavers from Appaloosa Plains? If so, those are made of four rotated cornerpieces from the Stone section of the floor tiles. They look like this:
To rotate the tiles, hit the < and > buttons.
You
do need to place a register for the pet shop to work, but don't worry; the premade pet shop comes with one already. If you purchased the Limited Edition version of Pets or downloaded it from the Store, the building should appear in your Building Library. You can reach the community lot section of the Building Library from Edit Town by clicking on the tab with a picture of a park bench on the toolbar. If you can't place the pet shop on your chosen lot, the lot is either too small or zoned incorrectly. If it's the latter, go out of the Building Library, click on the lot and click the Change Lot Type button to change it from a Residential Lot to any kind of Community Lot. You should then be able to place your pet shop.
I hope that helped you. Since you're a newbie to PC Simming, I'd reccommend going through
Carl's Guides if you haven't already. There are tons of great tips in there. Happy Simming!