Start with the simple things... How is the level (and condition) of the Sele fluid? (The level should be checked with the key at MAR, after the pump has primed).
Can you hear the pump run?
After it's warm, will it go into all gears with the engine off?
2nd/4th/reverse are all selected in the same direction from the neutral position (think of the gate in a standard manual TS gearbox). So, it could just be a re-calibration required (maybe a friendly member nearby can do it for you?), or possibly a range sensor.
I looked at one the other week than (amongst other things) was refusing to select 1st/3rd/5th. It turned out to be dirty pump relay connections and/or the 20-way Sele loom connections (which were also dirty). You might have a similar issue.
The pump relay is one of the ones in front of the battery. From memory, it's the front one in the 3rd column, but they can be moved around. To make things doubly-confusing, it can either be a red one or a black one.
You might be able to locate it by it clicking in time with the pump running (or not). If in doubt, clean them all!
If all that fails, you really need to get it hooked up to AlfaDiag or FiatECUscan.