-Unzip all the files in into "C:\Program Files\THQ\Company of Heroes\WWII\Data\Scenarios\mp\"
(if you don't already have a "mp" folder in your "...\WWII\Data\Scenarios\" folder, make one!)
-Create a shortcut to "Company of Heroes.exe" if you don't have one.
-Right-click on the "Company of Heroes.exe" shortcut and add " -dev" to the end of the target.
"C:\Program Files\THQ\Company of Heroes\RelicCOH.exe"
"C:\Program Files\THQ\Company of Heroes\RelicCOH.exe" -dev
(make sure you have a space before the -dev and that it is not in the quotation marks)