Inside Apple Mail, to set Signatures, go to theÂ MailÂ menu and chooseÂ Preferences. Select theÂ Signatures icon,Â second from the right.
Make sure the "Account" is highlighted on the leftmost panel. Don't create the signature when "All signatures" is highlighted, though, as that doesn't seem to work. Instead create your new signature by pressing the small plus sign on the lower left of the middle panel. Edit the text of the new signature in the rightmost panel.
At the bottom of the dialog box is a drop menuÂ , "Choose signature." Make sure the Signature you just created is chosen. Don't choose "None," "Random Order" or "In Sequential Order."
Now, finally, close the Preferences window by hitting the red button in the top left corner, or by pressing Apple-W. Quit mail (Main menu, Quit, or Apple-Q), and relaunch it.
You should be able to choose the new signature when creating a new message now.