CompuServe Messages

I am trying to use amiga programs that I downloaded on my PC on the Amiga at my school. The Amiga has crossdos. I got it to work once but I must have forgotten what I did. What might be the problem. I'm told that my disk isn't a dos disk. What I thought I did was go into Text Conversion for DF0: but when I tried it later it told me that it was not available. Any help would be appreciated!