The invalid magic refers to the magic number stored in every file. It determines the type of the file is some sort of header filled with info. So basically it looks like your file may be invalid or corrupted.
Could you re-download the file on your slug and redo a ipkg. A way to look at ipkg under *nix is normally doing the command ‘file’ followed by the file name.
Post back your results and we will take it from there …