To verify the DNS settings relating to DKIM for your Outbound SMTP service first open the "Sender Domains & DKIM" configuration page for your service, found under your account in the DuoCircle client area.  DKIM signing is performed on a per-domain basis.  Click the blue magnifying glass button (left) under the "DKIM tools" heading for the domain you wish to verify:

this will open the DKIM settings for the domain, where you will find a link to the MXToolbox DKIM validator near the bottom:

Understanding the MXToolbox Verification Result

No DKIM Records exist

If the MXToolbox DKIM validator returns a record looking like this:

It means that MXToolbox could not find a matching TXT record for the active DKIM key in your domain's DNS.  If you have already configured the necessary TXT record with your domain's DNS provider the change may just need more time to fully propagate through the DNS system.  If more than 24 hours have passed and you still receive this result from the DKIM validator please double-check that your TXT record has been configured properly.  It is very important that the TXT record name match exactly* the "Record Name" provided for the domain on the "Sender Domains & DKIM" page of your client area (see above). 

* Some DNS providers will require suffixing the Record Name we provide with your domain name.  To accomodate this append a dot and then your domain name to the provided Record Name. 

Success / Valid Configuration

If the MXToolbox DKIM validator returns a record looking like this:

it means your domain's DNS is properly configured for DKIM.