If you don't find any ready-to-use model for Proteus, you can make your own ISIS device, either by using some of the AD9850 firmware available on the web, or by using some PSPICE model (which perhaps will be more unlikely to exist for a not so simple device such as this one). It is not a complicated procedure, although laborious. There are tutorials on youtube explaining how to do that step-by-step in both modalities. It's fun, and also a good chance to learn how to create models for that CAD tool.