Except for using Letter instead of Double and hard-coding the number of rows and columns, your code is exactly the same as the example given in the Swift docs, so I don’t see any immediately apparent reason why it wouldn’t work. I agree with Chris that we need to see some more code.
@Chris_Parker, check out the Subscripts page in the Swift documentation for how to do custom subscripts. They can be pretty useful. In fact, I used one in this answer I posted just the other day.