If you cannot connect your Salesforce account to Geckoboard, first check that you're using either Enterprise, Unlimited, Developer, or Performance editions of Salesforce (though we don't support custom domains or sandbox environments) and that API access is enabled.
If you still can't connect, check your Session Settings. If the option to lock sessions to the IP address from which they originated is checked, the connection will fail due to the fact that Geckoboard authenticates and then queries using a different set of IP addresses.
Instead, set session settings to only Lock sessions to the domain in which they were first used.
If you're still having trouble connecting to your Salesforce account, get in touch.