The adapter I'm using (also from Radio Shack) is a 1amp with selectable voltage. And, I run the MSA on the 9v setting.
From the MSA. . . the "+" lead goes first to my on/off switch then from the second terminal of the switch to the "pin" connector of the power jack. The "G" lead of the MSA goes directly to the "shell" connector of the power jack.
The plug and jack are both size "N." With the plug set to "+ tip." It's one of those typical Radio Shack ones with the reversible polarity.