Re: 2-button footswitch
A standard two button switch needs three leads (common plus a return for each switch). I'm not sure how the Fender switch accomplishes that without looking inside. It may have some kind of relay switching that will only work with Fender amps.
Bottom line, it would seem you bought a switch that won't work with your amp.
By the way, the Marshall jack is like a stereo jack, typically called a tip-ring-sleeve or TRS jack. The common is wired to the sleeve, while each switch is wired through the tip and the ring.