When you run wxLauncher, you need to select the location you installed Freespace2/GoG to. Here's the config on my system:
(yes that's in Linux, but the screen should mostly look the same on Windows.
Select the "Basic Settings" tab
Enter the Freespace2 install directory into "Game root folder" (and there must be a bug in wxLauncher, it's supposed to show a "browse" button to let you select the folder
Select FS2 Open 3.7.4 for the "FS2 Open Executable"
Set your desired resolution, Audio playback device and Joystick (if you have one) in the same tab
Next you want to run the original campaign with updates, so you need to select the mod called "MediaVPs 2014"
Select the "Mods" tab in wxLauncher
Find "MediaVPs 2014" in the list & click "Activate"
Click on "Play"
If you continue to have issues, please post some screenshots of your wxLauncher "Basic Settings" and "Mods" tab and we'll help you out.