The button is only disabled if the sqlnet.ora file does not exist in the location expected. Double-triple check the file's location. Refer to Oracle's documentation regarding the search order for the files (section 5.1) https://docs.oracle.com/cd/B19306_01/server.102/b15658/cnfg_net_srv.htm#UNXAR213. Also, click the '...' button beside the selected client on the login window. Post a screenshot of what you see in there.
Michael