Threshold in Ps for iPad

Its not on there yet.  Though Michael has a good hint to help you.  But be patient, a lot of features are coming soon 

Threshold (as an adjustment layer or even just an adjustment) is not yet supported in Photoshop for Mobile. However, youcan get a very close approximation by adding a Black and White adjustment layer, then adding a Levels adjustment layer with the left and right "Input" settings pulled together as close as possible to the levels value you want. Here's an example: