Arrays like this are a little more complicated, SourceMod doesn't have a way of working out the offset for you.
First you need to get the offset of m_iAmmo using FindSendPropOffset (or FindDatamapOffset in the case of datamaps).
Then we look at the sendprop dump to work out the offsets to each row.
Sub-Class Table (3 Deep): m_iAmmo
-Member: 000 (offset 0) (type integer) (bits 10)
-Member: 001 (offset 4) (type integer) (bits 10)
-Member: 002 (offset
(type integer) (bits 10)
The offset x section shows us that each line +4 from the previous.
So to get 006 we'd have to do: offset (from FindSendPropInfo) + (6*4)
Then we can use this new offset with the ent prop functions. Usually this would be SetEntData* (slightly confusing as the data in this case has no relation to Prop_Data/datamap types).
In the case of m_iAmmo each ammo type used in the game occupies a different in this table. This is entirely game specific so you might need to play around tracking the values while shooting guns to figure out which one relates to what.