Download the YT component and double-click it. This should activate foobar and the component will be added to the components list. After that just press OK on the installed components window. Foobar will restart.
The next part is trickier, because you have to place the installed components somewhere on the Foobar window. The first step is enabling the layout editing (View -> Layout -> Enable layout editing mode).
I have no idea how your interface is organised, so I'm not able to be very specific. In my installation I have created a new tab in a window that displays album list, selection properties and a few other things (right-click on any of the tab headers and select "Add new tab"). Creating a new tab gives you a blank surface to work with. Next I added in it a horizontal splitter (right-click -> Add new UI element -> Splitter(top/bottom). Then in the upper part I have added the Youtube search component (it should be accessible in the same UI element list), and in the lower part the YT video.
If you want to add several components in one window, you have to divide it's surface before you do that. Once you select UI element, it will take the entire available surface and to add something else first you will have to remove it, split the window and add new elements to the parts of the window. That's why it helps to know upfront how you want the GUI arranged.
When done don't forget to turn off the layout editing mode (the same way as turning it on) and restart foobar. After that you should be able to do searches in the newly added component and the results will display in the playlist window.
I don't use youtube playlists so don't ask me how to get that into Foobar.