Open your HTML email signature file in your web browser.
Press Cmd + A to select the entire email signature, then press Cmd + C to copy it.
IMPORTANT: Do not use your mouse to select the area, only use Cmd + A as you may miss out on grabbing some of the code.
Open Polymail for Mac, then from the menu bar select Polymail then Preferences.
From the Preferences window you will see four tabs on the left hand side, select Accounts.
From your Accounts window you will see where to insert your email signature. Click inside the text field then press Cmd + V to paste your copied email signature.
Now that your signature has been pasted into your settings and is displaying correctly all you have to do is close out of preferences and start emailing! Compose a new email and you'll see your email signature inserted each time you send an email from that account.