Lr mobile does not provide support for adding or removing keywords in the current release.
Edited my typo
There will be many different use-cases and LR Mobile will be constrained by the little processor in a tablet, but I do hope features such as keywording, and selective display or filtering of a collection on the tablet, to display only images of relevance to the immediate work, will be in a near future release. Then the kind of work that's needed for organizing, selecting, and preparing images for work back on LR Desktop can be done while away
For my work I would not use the image editing tools too heavily since the trusted work has to be done on a calibrated monitor with a better gamut than the tablet, so I'd put more value in library tools, than image editing tools.
Andy, please add your vote and opinion to this feature request in the official Adobe feedback forum: Lightroom Mobile: Ability to do keywording on the move. That will (in a small way) make it more likely that Adobe will make it a priority.