1-Make a backup of the files mentioned below of the original RTCW game, 
because they will be substituted:
a)-cgamex86.dll
b)-qagamex86.dll

2-Unzip all files to RTCW directory
3-Play the addon with shortcut "Red Alert.ink"
4-After you finish the addon, replace the "files.dll" in your
original game.

Note: The shortcut only will work if the game be installed in C:\, except you it should 
change the properties of the shortcut indicating the new way.