You need to ensure that there is a Windows account called IIS APPPOOL\DefaultAppPool
on the machine. Run Computer Management on the machine,go to Local Users and Groups, and look at the properties in IIS_IUSRS
.
If there is no account there called IIS APPOOL\DefaultAppPool
then that is why you cannot add a login to SQL Server. You will only have this account on your SQL Server machine if you are also running IIS on that machine, as IIS APPPOOL\DefaultAppPool is a local account.
This link http://forums.iis.net/t/1174325.aspx seems to be very similar to your problem. There are some tips on how to solve the problem, including one at the very end which looks important.