At the bottom of your Blend toolbar (the vertically-oriented list of tools and controls -- on the left, by default), there is a button that looks like right-facing chevrons/arrow.
Clicking this will expand the list of controls at your disposal. You can browse, but by default your cursor will now be in a search box. Type the first few letters of your control name, and you should see it appear. Click it.
Now, at the very bottom of the toolbar, should be your control. Hover your mouse over it to see the name and description, if any.
In the Objects and Timeline pane, select the window or panel to which you want the control added, and then double-click that button at the bottom of the toolbar.
Alternatively, you can drag-and-drop the control into your form or into the Objects and Timeline pane.