Interesting question as mine are approved automatically.. I'm sure I haven't toyed with the settings but i'll have another look.
EDIT:
Found it
Under your channel banner on the right hand side you'll have a edit option for the channel.. it gives you two options
http://gyazo.com/d16fb9c3712d765f2d8337c4c38fa5c0
click "discussion"
at the bottom you can choose "display automatically"
cheers!
