You don't need to edit the configuration file to do what you want. Tunnelblick's "Route all IPv4 traffic through the VPN" checkbox will activate that OpenVPN option.
To change the setting of that checkbox:
- Launch Tunnelblick.
- Click on the Tunnelblick icon in the menu bar.
- Click on "VPN Details...".
- In the window that appears, click on the large "Configurations" button at the top of the window.
- Click to select the configuration(s) that you want the setting to apply to. You can use "Command-click" and/or "Shift-click" to select multiple configuration, using the standard OS X (and macOS) conventions for selecting multiple items.
- Click on the "Settings" tab on the right side of the window.
- Click so a check appears in the "Route all IPv4 traffic through the VPN" checkbox.
The settings are applied immediately.
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If you want to edit the OpenVPN configuration file:
- Click on the Tunnelblick icon in the menu bar
- Click on "VPN Details...".
- In the window that appears, click on the large "Configurations" button at the top of the window.
- Click to select the (one) configuration whose OpenVPN configuration file you want to edit.
- Click on little "gear" icon that is below the list of configurations on the left side of the window. (This "gear" icon below a list is also an OS X convention, as are the "+" (add) and "-" (remove" buttons to its left. They add and remove configurations.)
- Click to select "Edit OpenVPN Configuration File..." (see the note below if that does not appear).
That will open the file in TextEdit (the standard OS X text editor). Make your edits and quit TextEdit (menu command: TextEdit | Quit TextEdit). If asked, confirm that you wish to save changes.
Note: If "Edit OpenVPN Configuration File..." is not available but "Examine OpenVPN Configuration File..." is, that means that the configuration is a "Shared" or "Deployed" configuration. The configuration files for them cannot be edited. You can convert a "Shared" configuration to a "Private" configuration, edit the OpenVPN configuration file, and then convert it back to a "Shared" configuration. the "Make Configuration Private" and "Make Configuration Shared" commands are also on the "gear" menu, and are applied to any/all of the configurations that are selected in the list.
PS: So you're not an OS X guy either, right? (iOS is the OS for iPhones, OS X is the OS for computers. But "OS X" will change to "macOS" this fall when the next version, "Sierra", is released.)