I followed these instructions from assembla and was able to clone a repository from there:
The only thing of note is, that I had to create the following entry in ~/.ssh/config:
followed by using the following clone command:
git clone "assembla:username^reponame.git"
From your snapshot, it would appear that you are using Windows, therefore the location of the private key (id_rsa_assembla or whatever you are using) as well as the location of the ssh_config file will depend on how you installed git. See this question.