Have you imported text files successfully before? If not it could mean not all features were installed on your MS Office installation. Try reinstalling and make sure all import/export features are installed for MSAccess.
If it is only a problem with this particular text file, I don't have much to offer other than look at the text file and make sure there isn't junk, ie, headers/footers/graphics. Maybe saving txt file under a different name and trying to reimport...sorry can't be of more help.
try searching this forum for Search Key Not Found or variations thereof.
Good Luck!