This can only be done by a project administrator if the Paratext project is shared.
The FLEx project has to exist already, but the project cannot be open in FLEx.
In Paratext, go to Project > Project Properties and Settings (shortcut Ctrl-J). In the Associations tab, click Choose for Associated Lexical Project. Choose Fieldworks Language Explorer and you should be able to choose the FLEx project. If the project is open in FLEx at that moment, Paratext will say you have to close it in FLEx.
If the language IDs of the FLEx project and the Paratext project do not match, Paratext will show a blinking exclamation point. If you mouse over this symbol, you will see "Language ID is not valid for the associated Lexical project."
What has to match is the language ID of the Paratext project, and the internal code of the vernacular writing system in the FLEx project. The language ID is underlined in these two screen shots; the first is of the Paratext project properties and settings, the second is of the FLEx writing system properties. You see in this example the language name is quite different in Paratext than in FLEx, but the language ID does match. If you have advanced information (like the Latin script specification in this example), you have to have the same specification in both projects.