Let's see if I got this right:

I have installed Atlas April CTP. Now I wanted to use Atlas Control Toolkit. Something is missing because IntelliSense wont recognize all the tags and when I try to build my app I get an errormessage about PopControlExtender (Unknown Server Tag: act:PopUpControExtender). I extracted the toolkit files and copied the AtlasControlToolkit.dll to my app's bin-directory and made an reference to it. I also registered the assembly in my aspx:

<%@ Register Assembly="AtlasControlToolkit" Namespace="AtlasControlToolkit" TagPrefix="atlasToolkit" %>

Then there is also ScriptManager definition in my aspx as well:

I also added ACT to my toolbox (I'm using Visual Web Developer) and whenever I try to drag an control to aspx-page, I get an error message: "Control cannot be created because Visual Studio cannot find the control's type in the control assembly". 

So there you go. What steps do I need to make in order to make this thing work ?

The prefix needs to match what you've registered.  Your registration is using the prefix "atlasToolkit", so "atlasToolkit:PopupControlExtender" should work.

The other error you get is something people sometimes see on first installing, I haven't been able to figure out why.  In almost every case restarting VWD does the trick.

Well, no luck. This is what I registered:

<%@ Register Assembly="AtlasControlToolkit" Namespace="AtlasControlToolkit" TagPrefix="atlasToolkit" %>

and this is what I use in the code:

<atlasToolkit:PopupControlExtender ID="pceCalendar" runat="server">
.........

and this is what I get:

Error 102 Element 'PopupControlExtender' is not a known element. This can occur if there is a compilation error in the Web site.

When I look at the IntelliSense menu, I can only see one item related to ACT: atlasToolkit:ReorderList. Should I see more ?

I finally figured out ...

After the first install there seems to be another file to be installed, AtlasControlToolkit.exe. Installing  this another file extracts a dll which has to be copied to app's bin-directory. Name of this file is Microsoft.AtlasControlExtender.dll.

So my bin-directory contains actually three files:

AtlasControlToolkit.dll

Microsoft.AtlasControlExtender.dll

Microsoft.Web.Atlas.dll

And I have also made references to those dlls

Now I got the IntelliSense working and my PopUpCalendar too.

i'm faceing the same problem,  after i installed the toolkit it worked well ,but later one sometime it works and sometime it doesn't i dont know what is the problem with it, sometime i just close the visual studio and open it again it works, after sometime it gives me the error again !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!