This article describes why the FLX 3401 is not visible in the list even though you have the FLX 3401 extension card.
I have the FLX 3401 extension card, but I can’t see it in the list.
The FLX 3401 is deprecated and has been replaced by the FLX 3402 since July 2016.
The FLX 3402 can do exactly what the FLX 3401 does, and has been improved to measure 4-20mA & 0-10 VDC signals.
However, both cards still have their installation guide document in the Downloads & Documentation section.