The problem with the code at present is that you're referencing a single 'motif' when in fact you need a separate one for each letter.  Now you could create an array of motifs - one for each letter; but when you start using so many nested arrays it's worth considering creating another class...

So here to simplify matters I'd create a new class, perhaps called 'Letter', which includes a 'motif' property and uses this to draw Cells in the shape of the desired letter.  You'd then be able to create separate Letters at each point and simply update the motif property of the current letter when a key is pressed.  That should also allow you to get rid of that 3d array (such things make my brain ache).

That's a nice suggestion. To be sure (I want a 11x5x5 grid of cells), should it go very roughly like this: