Yes, you must be a project administrator to do this. It is a project-level global setting which is always restricted to administrators-only. All users that are on the project, then need to have FieldWorks installed and must have the associated project if they want to use the dictionaries. It must be the same project (which in FieldWorks, I think just means it needs the same name if it’s located in the FW default project location).
I realize that it being project-level doesn’t make much sense, currently, but the long-term goal is to get all of Paratext’s lexical features to feed into FLEx directly instead of using the current Paratext lexicon. Things like the Interlinear, Morphology, maybe the whole Word List, etc.