Open an Explorer-Window. Go to “Tool”->”Folder options…”->Tab “View” and choose the option “Show hidden files, folders, and drives”.

Now go to:
(Windows XP)
C:\Documents and Settings\[YOUR PROFILE NAME]\Local settings\Application Data\Lookeen\[YOUR OUTLOOK PROFILE NAME]
(Windows Vista or Windows 7)
C:\Users\[YOUR PROFILE NAME]\AppData\Local\Lookeen\[YOUR OUTLOOK PROFILE]
Store the folder “Index” and paste it to the same location.