You need to hold down the mode button until it switches to li4po setting.
Whoa, careful here! The stock batteries for the Zion and Air are IMR, not LiFePO4. It can be dangerous to charge any Li-Ion battery improperly. Although charging IMR with the LiFePO4 setting might not cause serious problems, it also won't work.
@phattpiggie tthanks.
What about the modes etc? Can you take a pic of the front of your D2 while its charging? I don't mind charging slow as long as I'm charging SAFE haha.
In normal use with a D4 and D2 etc can you charge AAA and AA batteries, then pop in an 18650 and change no settings? Its just smart enough to charge everything safely on its own? I charge a lot of AA and AAAs so I want to feel confident in my charger. I know its safe, I just don't feel confident that I'm using it right. I'll be looking for a PDF of the manual later.
The Nitecore D4 is a versatile charger (see
this thorough review) but it's pretty slow. The rate depends on the number of batteries you're charging, and even if it's just one battery, the rate is only 750mA. That means it would take about 3 hours to charge one Zion battery from completely drained. Of course nobody would drain them completely, but almost certainly you'll be charging a pair, so the time doubles. I'd expect at least a four hour charging time. It's much worse if you charge three or four batteries because it drops into low current mode (300 mA or 150 mA). Four completely drained Zion batteries would take about 15 hours! The D2 is a bit slower than the D4 for a single battery, but faster for two batteries since it uses 500 mA per battery. That's still pretty slow, 4.5 hours or flat Zion batteries.
The D2 and D4 can tell NiMH and NiCad from Li-Ion, but if you want to charge LiFePO4 then they have to be switched to the correct mode as
seaofgreens mentioned. It's dangerous to charge LiFePO4 in IMR mode.
It's normal for the charger and batteries to feel warm while charging, but not hot. You should be able to handle them easily. If you can't, something is wrong and you need to determine whether it's the batteries or the charger.
Edited to clarify IMR-LiFePO4 difference.