I keep getting this error that says "Unable to open all mix files, please insert a C&C CD or see readme.txt for details."

I assume that it says this because I reinstalled my Red Alert to a different directory that isn't the default. (Just to note, I also reinstalled this program after I reinstalled Red Alert, and I do have the CD in the CD-ROM drive.) Is there any way to get this program to work if Red Alert isn't installed in the default directory? Or what is the problem if it isn't the default directory?

And where is this readme.txt file it asks for?