Ah, the good old days. Imagineer aka Smartsketch was the best Mathcad feature ever. It is still a great application. I still use V4 far more than Mathcad - very often to illustrate calcs in Excel!
The CEO at PTC should be sacked if an equivalent drawing application is not supplied with the next version of Mathcad. (Assuming that a new version is being planned rather than just continuing to sweat V14).
The free version of Solid Edge 2D is one of the best free applications ever. It looks and works like Smartsketch and is obviously related at some time in its past. I would suspect it will not work with Mathcad. However it should do the same job if you can manage only with dumb drawings.
The only problem with the current free version is the extremely annoying MS Office style 2007 ribbon bars.
There are subtle, and not so subtle, differences between Visio and Smartsketch. Note, however, there are no current plug-ins for Mathcad for either one, and haven't been for about 8 years.