ACT for Web Impersonation Account - Best Practices
The ASP.NET account is the Windows Server internal account for IIS and ASP.NET. Usually that one is set up successfully by the Windows Installer when IIS and ASP.NET is installed.
The ACT! Impersonator account needs to be a Windows account that has Local Administrator access to both the machine that is the IIS server as well as the ACT! Database SQL server. I usually don't set up ACT! for the Web where IIS and SQL Server are on different machines so I usually just create a Local Administrator account on the one machine and it works. However, since you have two machines involved, what I would recommend (and you did this) is to create a Domain User account and then add that account to the Local Administrator's group on each machine. This works UNLESS one of the machines involved is a (or the only) Domain Controller, in which case there is no Local Administrator's group and you have to set the account up as a Domain Administrator for it to work (and if IIS and SQL Server are both on the one and only Domain Controller then again you have to set the account up as a Domain Administrator since there are no Local Anything accounts on a Domain Controller).
When you go to add the data base to ACT! for the Web you now need an ACT! Administrator ID and password. If you successfully set up the Impersonator account you proved you had authority over IIS and Windows. Now you need to prove you have authority over the ACT! data base to add it to the Web site. And it is at this point that the access to the SQL Server (ACT! Data base) machine is tested so it sounds like this is the error you are seeing.
If you have set up the ACT! Impersonator account successfully you should be able to add the data base to the Web site successfully. One thing to test is to log on to the Terminal Server as the Impersonator account and then see if you can open ACT! and access the data base on the ACT! Database server. If you can't you will probably get a better error message there.