In older versions you could select a message (using the keyboard) and click the context menu key on the keyboard (replicates a right click) - the menu had an option to open the attachment. This was removed from outlook 2010, much to the annoyance of many users.
AFAIK, there isn't a shortcut to get into the attachment field in the message list - tab won't put you into the reading pane.
If the message is open, tab (or SHIFT+TAB if you need to go backwards) will get you to the attachment row - then use the arrow key to select messages. Enter to open in their application, select to preview in the reading pane (if supported).