VS 2010 NormalSlider Control Does Not Show Up In The Toolbox

I made a UserControl called SliderUserControl which contains two labels and a WebSlider (third party slider control) in a table. The labels left and right of the slider should give the value of the slider and (Maximum - value) of the slider respectively. So if the slider goes from 0-100 and is at 30 now, then the control looks like this:

Code:
30 ----|---------- 70
I accomplished this with some simple javascript, and it's all working just fine:

asp Code:
<%@ Control Language="vb" AutoEventWireup="false" CodeBehind="SliderUserControl.ascx.vb" Inherits="F1TimeTrials.Controls.SliderUserControl" %>
<%@ Register assembly="Infragistics35.Web.v10.2, Version=10.2.20102.1011, Culture=neutral, PublicKeyToken=7dd5c3163f2cd0cb"
[code]....

I said, this works fine. If I use a Register directive that points to the SliderUserControl.ascx source then I can use it on my page and it works.However, I actually need three different kind of sliders, each slightly different from this 'base' slider. For example, one will work with integers, another will work with float point values, etc. So what I want to do is create three classes that inherit SliderUserControl, and customize it from there. So, for example, a slider that goes from 1 to 11 with only one label (without the extra second label on the right) could look like this:

vb.net Code:
Imports Infragistics.Web.UI.EditorControls
Namespace Controls
[code]....

And now I'm a little stuck... I can build this project, but the NormalSlider control does not show up in the toolbox as I'm used to from windows forms projects. I thought well, that's probably different for ASP.NET, but then I remembered that it DOES work for a different control. I have a class Grid that inherits a third party UltraWebGrid control:

vb.net Code:
Imports Infragistics.WebUI.UltraWebGrid
Namespace Controls
Public Class Grid
Inherits Infragistics.WebUI.UltraWebGrid.UltraWebGrid

'...
End Class
End Namespace

When I build the project now, the Grid control does show up in the toolbox! So that's a bit strange..However, I googled the problem of course and I figured out how to get a custom control (class inheriting a control) onto a page, and apparently I can use another Register directive where I need to specify the Assembly and the Namespace. As far as I know, the Assembly is just the current project (the control is not in a separate project) called F1TimeTrials, and the Namespace is either Controls or F1TimeTrials.Controls. So I tried both of those, for example:

asp Code:
<%@ Register TagPrefix="uc" Assembly="F1TimeTrials" Namespace="F1TimeTrials.Controls" %>

Now, I can type "<uc:" and Intellisense shows me the NormalSlider control. Great! However, when I place it on the page, it does not show up during design-time.

asp Code:
<%@ Page Title="" Language="vb" AutoEventWireup="false" MasterPageFile="~/Site.Master" CodeBehind="SliderTest.aspx.vb" Inherits="F1TimeTrials.SliderTest" %>

<%@ Register TagPrefix="uc" Assembly="F1TimeTrials" Namespace="F1TimeTrials.Controls" %>
[code]....

There are no warnings or errors or anything, but the control isn't there.When I run the project, it throws a null-reference exception in the Page_Load event of the NormalSlider class. Apparently, Slider (= the WebSlider third party control on the base class) is Nothing. I don't get it... How can it be Nothing in the Page_Load event? Should I call some method that creates the control in the base-class first?