Your amp only has one power supply port, designed to take the OUTPUT* plug of a wall wart AC/DC converter (ie: the PX-2g).
jea48 is correct! The PX-2G is a transformer step down 120VAC to 9VAC, it's not a wall wart AC/DC converter.
The, "AC IN" labeling is an error and the company should made aware of THEIR confusion (AS IF they care, over there).
*AC in / DC out, of course.
No, the "AC IN" labeling is not an error! The Line 6 Micro Spider amp will convert the 9VAC to DC internally.
BTW, a wall wart AC to DC converter may work as well, but after the Line 6 Micro Spider amp's internal rectifier, a 9VDC input will output much lower DC voltage than a 9VAC input. You need 12.7VDC input to get the same DC voltage after the rectifier of a 9VAC input.