You should consult the OpenVPN docs for it, but I think it works as follows:
The file is a plain text file that contains the username, a LF character (0x0A), the password, and another LF. It should have a ".key" extension and should go in xxxx.tblk/Contents/Resources.
You reference it in the OpenVPN configuration file using
auth-user-pass NAME_OF_FILE.key
If Tunnelblick won't let you install a .tblk with such a file (or complains about the filename in the auth-user-pass line of the configuration, you can try to do it manually. Do that by creating a .tblk configuration without the file (and including "auth-user-pass" without a filename) and install that as a private configuration.
Then go in manually to add the file into the Contents/Resources folder of the installed .tblk. (Private configurations are in /Users/YOUR_USERNAME/Application Support/Tunnelblick/Configurations.) Then add the filename to the "auth-user-pass" line in the OpenVPN configuration file.
Please report your results here, to help others with the same problem.