Installing Mergetastic into your Bitbucket workspace is extremely simple. Start by clicking the "Try now!" button on the top-right corner of the page and follow through on the modal that pops up.
That will take you to a page that looks like:
Figure 1: The Mergetastic installation screen
Note that dropdown in the installation screen. It allows you to choose the workspace Mergetastic gets installed to.
And that's all there is to it!