Might not be your situation, but sharing my notes Justin case they might be helpful.
I had some trouble editing my tnsnames.ora & sqlnet.ora after installing TOAD on a new windows10 laptop from my employer. This turned out to be a permissions issue.
Even though TOAD had installed correctly (thanks to software that automatically elevated its privileges for the install), I had to run it elevated to access my sqlnet config files.
For me, that meant when running TOAD, first Shift+RtClick, then select Run Elevated. Click “OK” to “I need to run an Application”.
Then to get the LDAP drop-down working, I had to edit both the sqlnet.ora and ldap.ora (even though they were already fine). I readded all the ONN servers into the LDAP, ordered them correctly, then saved that file. Then re-ordered the NAMES.DIRECTORY_PATH for the sqlnet.ora and saved that. And hey presto, my LDAP drop-down appeared.