Not sure if it will help but the only time this happens to me is when I'm trying to edit a recordset and not a table. Does it happen to you if you click on the table name on the tree control on the left and then press CTRL+ENTER? Is it a derived table?
Even if you open a table you may not be able to edit it provided that the given table has no primary key. Constructing corresponding SQL statements manually will do the job, though.