windows->preferences no fitnesse Configurations right click not fitnesse option as well
There can be many reasons for such an error.
Try starting Eclipse with -clean parameter. Then look in your logs, usually at workspace/.metadata/.log. See http://wiki.eclipse.org/FAQ_Where_can_I_find_that_elusive_.log_file%3F
If all else fails, try re-installing Eclipse into a new folder. Simply rename the old Eclipse folder to keep it for a backup.