Solution is to upgrade to 0.99.82 or higher, the bug has been fixed.
I have read up on the forums and seen videos that show 2 new ways to edit collision masks... yet neither is working for me.
1) Select sprite.
2) Go to Animator tab
3) Right click, launch explorer.
4) I am taken to C:\Program Files (x86)\Scirra\Construct\Images
5) There is only a blank "stub.txt" here.
1) Open picture editor.
2) Click Collision Mask button.
3) ... and nothing happens.
Is the current build broken in these areas or have I setup something wrong? I'm an IT engineer so feel free to get technical if I need to fix something...