Hi there!
This is something that we’re currently looking into at the moment. From what we’ve found the cursor does seem to work in windowed. While its more of a workaround than a fix, please switch to windowed using Command + M.
Doing some more research on this, the issue itself looks to be related to accessibility permissions. For that you’ll want to go to System Preferences > Security & Privacy > Privacy > Accessibility. If Heroes of the Storm is listed here, make sure it is checked. If the game is not listed here, you can proc the permissions prompt by checking the Disable OS Keyboard Shortcuts option in the Mouse & Keyboard settings of the game. This would need to be done in windowed mode.
If that doesn’t help, please post the version of OSX that you’re on. Will provided further updates as we get them.