While the app is available at the Atlassian Marketplace, it can be installed only from this install link. Since Forge on Bitbucket is so new (it just left EAP and became GA), it is impossible to install Bitbucket Forge apps directly from the Atlassian marketplace. It is Safe to install from here, and will be available on Marketplace when Atlassian makes it possible.
By installing the app, you automatically accept our Terms of Use.