If your data source type is 'Database' or 'Association' then you can just add the XPath constraint to the grid properties. For a data source of type Microflow, you need to constrain your data in the Microflow itself. An XPath constraint is basically a constraint in a database query and doesn't make sense when using a Microflow as source.
Update: I see now that you're referring to $currentUser as a variable, but the variable for this only lives inside a Microflow, in this case you should use the token which is '[%CurrentUser%]'