There are two answers - pick up the symbol you need from another document (or mail message) and use copy / paste (ctrl C / CtrlV) to drop the at character into the address.
You may well have some debris trapped under the key-cap - you can take it off - clean it with an old soft toothbrush and put the key cap back on (my keyboard has the best part of a cheese roll hiding under the keys)
This link may help...
http://www.laptoprepair101.com/laptop/2007/03/20/key-fell-off-keyboard/
Good luck...
Peccavi
February 2010