selecting multiple lines 1

[mecram1]

This may be a dumb question, but some how I have switched off the ability to select multiple lines to perform a task. Now when I select a second line, the first is unselcted. How can I turn the multiple selection back on?
Autocad mechanical 2008

 

[Exxit]

http://usa.autodesk.com/adsk/servlet/ps/item?siteID=123112&id=2860959&linkID=9240617

ADT 2004