Hi abundantc,
Currently, this option is not available, you need to create accounts through the adCenter website.. If you had created the account as EN_CA, this is the default setting for all the campaigns that are created under this account as they will all inherit the same settings.
What you can do is create a new campaign and select the EN_US distribution. Now using this method will allow you to track the two different distributions seperatly and easily.
Doing this means that you will have to sign in to the adCenter website and select the Accounts & Billing tab and then click the Create Account button, don't worry, you will not be charged another $5.00 fee.
Ensure that you select the correct currency that you wish to distribute your ads (USD=US, CAD=Ca, GBP=UK) from the options available. Once you have done this, you can reconnect to your account through the desktop tool and now you will be able to create campaigns for your desired distribution. This can be done by selecting the Account button and allow the tool to authorize your user account, then add the new account that is available.
Thank you,
Brian H - MSFT