If a user is running a report is not able to see the employee drop down filter, this indicates that the user is set up with Client Restriction. What this means is that the user has one or more clients moved into the "Selected Clients" column in the Clients/Notes tab of the Employee Set up.

If the user should have access to the employee drop down filter in reports, and does not need to be restricted to viewing data for specific clients, then all clients should be moved to the "Available Clients" column in the employee set up for the user.