Basically, you should install spice-vdagent package on Mint. Maybe to use auto-install Boxes feature (the last time I played with Boxes there was possibility to perform automatic installation) and then fully update guest system and install mentioned package.
I had the problem with mint too. The cursor is there but you don’t see it. So try to get the cursor above the terminal icon and click so you open a terminal. Once you have the terminal open it is simple to install the updates and after a reboot you will see the cursor.
I never installed spice-vdagent.