Hitchhiker's guide to the galaxy
 

My dosbox won't let me type " and it's pretty important to advance in the game.
Can someone tell me how to fix it?

It's somewhere a 'shifted' key, on my layout (azerty be) it's shift+%.

..... or you can set your keybaord layout to your own with the command
keyb xx
For me it's
keyb be
For you ....... http://vogons.zetafleet.com/viewtopic.php?t=21824

Or set a key to " with dosbox keymapper ..... (-> dosbox manual)