1) You can use the oqpskmod function or the comm.OQPSKModulator System object (and the corresponding block) from the Communications System Toolbox. These tools will produce the symbols for you.
2) Alternatively, you can map the unipolar bits [0, 1] to a bipolar signal [-1 -1], using this command: >> bipolarSignal = bits*2-1; Then you can follow what you have been doing so far and all that remains is to scale the constellation to your liking, e.g., by sqrt(2)/2.
