If a Sales Rep is not on the Lead Owner filter drop down it is most likely that the Sales Rep has not been assigned a Sales Rep Office. This assignment will need to be completed by an admin user.
To check the office assignment of a sales rep go to their profile and scroll down to Sales Rep Settings. Ensure that the box is filled in for Sales Rep Office Assignment. If that box says "---select---" the rep needs to be assigned to an office or an Office Manager will not be able to assign them to deals in their office.