it would appear you don't have ASP.NET Ajax installed (but, not necessarily). There are several reasons you may be getting in all these troubles:
1. You are running Sitefinity 3.6 built against .NET 2.0 framework, while you have .NET 3.5 framework installed on your machine (if that's so, ASP.NET Ajax is already installed) - if this is the case, download Sitefinity 3.6 built against .NET 3.5 framework and your problems should be fixed
2. You are running Sitefinity 3.6 build against .NET 2.0 framework, while you have .NET 2.0 framework installed on your machine, but don't have ASP.NET Ajax installed. If this is the case, please go here:
and download ASp.NET Ajax 1.0 and install it.
If none of this helps or you find it confusing (which it is, I admit), please let us know following:
1. which version of Sitefinity you are using and if 3.6 against which .NET framework (2.0 or 3.5)
2. which version of .NET framework you are running
3. do you have ASP.NET Ajax installed and if so which version
4. which version of IIS you are running
5. which OS you are running
I am sure, we'll sort it out then.
All the best,
the Telerik team