If you're using the Paths panel to select points, you'll need to click on the object (on the canvas) after making the selection. Then you can use the arrow keys.
The reason for this glitch has something to do with "keyboard focus" being on the panel instead of the object. The developer of the Paths panel, Aaron Beall, is aware of this issue and is planning on releasing a fix for it, according to this recent thread:
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