After adding a new ChartMogul widget from your dashboard, enter your API Key. You'll find this on ChartMogul's API Admin page.
Click Connect. You'll now be asked to log into your ChartMogul account, unless you're already logged in.
Finally, you'll be asked to give Geckoboard permission to access your account.
To connect a different ChartMogul account to the one you've already connected to Geckoboard, see connect multiple accounts to data sources.