If your Freespace 2 directory has a space in it. (C:\games\freespace 2) rename your Freespace 2 folder to FreeSpace2. If your folder is already called Freespace2 Proceed to next step.
If you are using the SCP, first make sure you have the SCP patch. Then put all the Inferno VPs (including the patch) into a folder called 'Inferno', go into the Launcher, click features, click 'Select MOD', and select the Folder called 'Inferno'. Then click run. If you are not using the SCP then proceed to the next step. If you are then Inferno should now be working.
Ensure all of the Inferno VPs are in your Freespace2 directory, and ensure you have no files in your C:\games\freespace2\data\tables folder. Inferno should now be working.
If none of this works then ask someone else.