When trying to use the program I am receiving this error message: "Unable to pass commandline to previous instance. Dialog open?" I even tried to reinstall, and it didn't help.
This error message means MasterCook has an issue trying to use its files. Oftentimes it is caused by a corrupt Mastercook.INI file which is a configuration file the program uses. This issue should be resolved by resetting things as explained here: Resetting MasterCook
If you failed to delete the MasterCook 15 folders after uninstalling and before reinstalling, the Mastercook.INI file will remain corrupt. This is why when you reinstall you need to follow these exact instructions: Reinstalling MasterCook 15