Clone the repository you've created (this is now your local repository). This creates a folder on your storage device.
Next, place your project's files in the same folder as the local repository (the same level as the ".git" folder). Your files are now in your local repository's working tree.
To actually "add" them to your local repository, you'll have to do two things: Stage the changes, then commit them. Staging means indicating which changes you want to commit (which can be changes to multiple files). Committing means adding the changes to the repository. You can do both of this via Source Control -> Commit Changes. If you want everything to be added, stage all the files, then commit.
You will be prompted to enter a commit message. This is where you describe what your commit changes about the project, such as "added enemy XYZ" or "fixed bug where player ABC when XYZ". Since you're more or less importing a project, you can just write "Initial commit" or "Project import" or something like that for this one.
Once your changes are committed to the local repository, make sure you have set up the remote repository (Bitbucket) as the project's remote repository and entered your credentials under File -> Preferences -> Plugins -> Source Control (Git). You'll then want to push the changes to your remote repository. You can do this via Source Control -> Push Changes. This adds all changes from your local repository to the remote repository. The reverse of this (adding all changes from remote to local) is Source Control -> Pull Changes.