You need to right-click in the table designer and pick Indexes/Keys:
Then a dialog pops up and you can add a new index to the list of indices (on the left hand side) and define it to be a unique index:
You need to right-click in the table designer and pick Indexes/Keys:
Then a dialog pops up and you can add a new index to the list of indices (on the left hand side) and define it to be a unique index: