If you've created the discounts in your Dashboard (Items-> Discounts) then they should be available almost immediately in your PoS.

If not, first thing to try is to force close the app and reopen it. It may not have synched up with the server for some reason.

On the app, there are two ways to access percentage discounts:

-Discounts should be in your left column near the bottom. You may have to scroll down a bit to get to it.

-In each item, you can apply a percentage discount by scrolling in the item modifier screen. If when you tap on an item it just shows up in the right column without popping up a modifier screen, you can just tap on the item in the right column to bring that screen up. Discounts are usually listed at the bottom of that modifier screen. (Locations of columns may be a little different for you depending on if you're using the tablet in portrait or landscape mode. The instructions above are assuming landscape mode!)

Dollar amount discounts won't be available in the item modifier screen, just the Discounts category on the left column.

More info is on Square's Discounts Help Page.