I have an Oracle tnsnames.ora file from my previous workplace. I want to pick it up with my newly installed PL SQL Developer on another computer. I have copied the file into ..O
You most certainly have a databases tab in sql developer (all versions I've used in the past have this). Maybe check again? Perhaps, you're looking in the wrong location.
On a mac, the preferences is under "Oracle SQL Developer" (top left) -> Preferences -> Database -> Advanced -> section called Tnsnames Directory is where you specify the file.
On windows (going from memory so might have to search if this isn't correct) Tools -> Preferences -> Database -> Advanced -> section called Tnsnames Directory is where you specify the file.
See this image