Hi Kinga,
You should be doing this within the BCC 10 filters, not within Mocha Pro. To do this, first download the trial version of the BCC 10 filters for your host … you can access them at https://www.borisfx.com.
Once you’ve downloaded and installed BCC 10, apply the BCC Pyramid Blur filter to a clip. In the effects control panel, you should see a button labelled launch mocha. Click that button to launch mocha from within the BCC Pyramid Blur filter. From there, set up create the mask shape that you need and close mocha (it should save automatically.) The result of this should be that you have isolated the blur to within the mocha mask.
Keyframes in mocha are in the Dopesheet tab of the mocha UI.
I hope this helps,
Peter.