Congratulation on your approval. In the process with a waiver approval, you should be receiving 2 letters.....
1) The approval notification letter -
which you already received2) The interview date/time notification letter-
which you are waiting on.........The interview date/time notification letter can take up to a month to receive after getting your approval letter. Additionally, the actual interview date/time may be set for a full month after receiving your interview date/time notification letter.
These things will vary depending on the consulate that you are going through. My fiance and I are going through Mexico's consulate and the above times are what we are currently expecting.........But, I think they are probably close to other consulates....Good Luck and don't worry! You are almost there!