You have not declared an instance of the Enum, you have simply declared it.
You need
public enum AppInstallType { msi, exe }
public class Application
{
//Properties
public string AppID { get; set; }
public string AppName { get; set; }
public string AppVer { get; set; }
public string AppInstallArgs { get; set; }
public AppInstallType InstallType;
public string AppInstallerLocation { get; set; }
}
if(InstallType == AppInstallType.msi)