You see that long rectangle on the side under the word "Search?". Using your mouse (or whatever pointing device you're using), move the arrow on your screen to over it and click. You should see a blinking line inside the box. Using your keyboard, type what you want to look for. Move the arrow over the button that says "Search" and click it.
Searching other parts of YTMND wiki
If your settings haven't been changed, YTMND wiki will only search for things on the main part of the wiki and not the YTMND: or Help: namespaces. To expand a search, click the checkboxes at the bottom of the results page and search again.