To install the compiler, download the following file and extract it into it's own folder anywhere you like:
Download Reflex Compiler 1.09 - (updated 13 Mar 2012)
Beta Track DCL
The following files you need to extract into the Database folder of your Reflex installation. Anyone who wants to run any of your beta tracks will need to install these files before they will appear in their track list under the Free Ride section. The files define 8 beta track slots. When you compile a track you can select which of these 8 beta slots the track will appear under in the menu. Then to run the track just select it from the menu. You can select any of the 8 beta track slots from the menu but if you haven't got a track compiled in that slot the Reflex will crash to your desktop.
You can download the Beta Track DLC files from here, unzip them into the /Database folder of your reflex installation:
You will find your Reflex installation in the folder 'X:\Program Files\Steam\steamapps\common\mx vs atv reflex' where X is replaced with the letter of the drive that you installed steam on, which will usually be 'C:'.
