The steps are;
1- Activate LaunchBar using your usual hotkey
2- Open the index using CMD-OPT-I (or click on the small cogwheel in LaunchBar and select "Index/Show Index")
3- Click the small '+' in the bottom left-hand side of the LaunchBar Index window.
4- Select 'Add Folder'
5- Navigate to ~/Library/Mobile Documents/ and choose that folder (**See below if you cannot see the Library Folder)
6- In the Options for the folder index you just created, set the 'Search Scope' to 'Search all Subfolders' and (optionally) 'Search For:' to 'Documents'.
This will give you a folder search called 'Mobile Documents' with all the iCloud documents in it.
However that will mean it lists a lot of rubbish you probably don't want. To change this you can either;
a) in Step 5 above Navigate to the specific app's folder that you are interested in e.g. ~/Library/Mobile Documents/[application-name]/Documents
b) Select 'Skip Subfolders' in the Index Options and drag all the folders you don't want into there.
I do the latter- putting all the games etc. that use iCloud in the exclude list and just keeping the apps (Numbers, Keynote, Simplenote etc.) that use documents so that new apps are automatically indexed rather than being excluded.
** if you can't see the library folder at all then just Google 'Show Library Folder'
; there's a tonne of articles about that.