I recently had a leak in my shower and replaced the Mixet cartridge which seemingly fixed the problem, but now the dial/handle seems unresponsive and isn't opening or shutting the valve. I can open it and close it by turning the handle screw with a screw driver, but that doesn't seem like a good solution. Any ideas? Did I do something wrong?

Did your cartridge look like this? http://www.plumbingsupply.com/images/mixet-ms-5at.jpg I would think you have not seated the handle on the spindle correctly. When you slide it on you rotate it until it slips onto a slot and seats a bit deeper, then tighten your screw.

That's what it looks like. I'll take it off and give it a try.

Seems like your handle is stripped. When you turn handle, it doesn't spin the stem itself. Get new handle in Home Depot.