Now with AutoCAD MEP 2009, you can have multiple catalogs listed in the search paths. This means all you have to do add your custom catalog to the search path. But be careful of one common pitfall. Users creating a custom catalog often take an existing catalog, make a copy of it and rename it and then swap the parts out with theirs. When they go to add the new catalog to the search path, they find that their catalog will only show up if it's first in the list. This is because of a duplicate catalog ID. If AutoCAD MEP has two catalogs with the same ID it will display the first in the search path list and ignore the rest with that ID. This can be easily remedied by changing a few characters in the catalog's ID. Take a look at the screen capture to the right.
As you can see I have changed the last 3 characters of the catalog ID to my initials (NMF). I also changed the catalogs description. Not changing the description will cause you to have two catalogs listed, both with the same name. This can be confusing for your users. I found that making these necessary changes is easiest done with a nice XML editor. The one used at right is XML Notepad. It's a free download from Microsoft's website. I recommend only getting it from Microsoft, which ensures you won't get any viruses. You can download it here, XML Notepad