Make sure to use git@bitbucket.org:{username}/{reponame} instead of any https links bitbucket might give you.
I was able to upload an existing repo I had by:
1) Uploading my SSH Public Key at https://bitbucket.org/account/ (i.e. the contents of ~/.ssh/id_rsa.pub) 2) Create a blank repo at Bitbucket 3) Adding a remote (git remote add bitbucket git@bitbucket.org:{username}/{reponame}.git) 4) Pushing to bitbucket (git push bitbucket master)
The most important parts were adding my SSH Public Key, and making sure to use git@bitbucket.org:{username}/{reponame}.git instead of any https links you might see on bitbucket.