As I understand from your query, you want to use 64 QAM in your model. For that, you can use rectangular 64 QAM.
To use this block just follow these steps
Library Browser>> Communication Toolbox>> Modulation>> Digital Baseband Modulation>> AM>> Rectangular QAM Modulator Baseband.
Alternatively, you can directly click on canvas and type rectangular QAM Modulator Baseband.
Select this block and change parameters according to your requirement by double clicking on the block.