We had a few problems because of a corporate firewall and an http proxy server. Only port 80, http traffic was allowed.
To do this in OSX the following steps were necessary:
1. Set the htttp_proxy variable in the shell:
if your proxy requires a username and password the command is:
2. Run the import using http instead of the git protocol:
script/plugin install http://github.com/adamlogic/showoff.git