During the installation process myApi tries to set your facebook application settings for you. However there are some know issues with get these settings correct. If you are having any problems at all using myApi first go to http://www.facebook.com/developers/apps.php and check you application settings by hitting edit settings.
Then go to the "Connect" tab on the lest hand side and look at your connect url and base domain. For my website http://www.myapi.co.uk the settings should be
Connect URL - http://myapi.co.uk/ (notice there is no www.)
Base domain - myapi.co.uk (neither http:// or www. should proceed your base domain)
Hope this helps