Sounds like the current contents of the column are dates in the format mmddyy. If so, Access will not recognize these strings as valid dates and so you can't convert them by simply changing the data type of the column.
You can add a new column to the table to hold the converted dates, run the conversion, delete the original column, and rename the converted column. Use an update query with something like the following in the Update To cell:
CDate(Left(YourFld,2) & "/" & Mid(YourFld,3,2) & "/" & Right(YourFld,2))