I assume you are talking about the new content link?
if yes:
Settings>General settings> Search Settings (as seen below)
It took me a while to find that as well.
if you mean just in general you should be able to see all content (I think it'll only show three months but am not sure) just by visiting the board you want.
A large number of forums have a forum board setting:
Default Thread Age Cut Off
You may specify a time period from which to display threads. Threads older than the age you specify will not be shown.
The choice range is from ?Show all threads? to various increments of periods of time.
I prefer ?Show all threads?. When I login to Piriform CCleaner Discussion, there?s only two pages of posts which stops at November 23. I want to be able to browse topics prior to that date.