Which OS, a Mac OS, Windows, Linux? What make and model computer?
I Googled "missing issetup" and didn't find anything to note, except a lot of 'suspect' web sites offering a free issetup file download. I would never trust those types of web sites.
So, is that the full message? If not, can you give us the exact error message please.
Do you have any other problems with this computer?
When I try to install Quicktime, I get an error message saying it cannot be installed because of "missing Issetup". Don't have problem installing any other software. What do I need to do?