They are (probably) good! First you must install the L4D2 Add-on support thing, which is in the library under tools. Then download the maps.

City 17: http://www.l4dmaps.com/details.php?file=5055
Death Aboard 2: http://www.l4dmaps.com/details.php?file=4861

For most people you just download the file, run it, and the game will automagically add it. You may also have to enabled it in-game under the extras screen.