Choose Site Actions / Site Settings / Site Features (under Site Administration)
or, if on a Publishing site, choose Site Actions / Site Settings / Modify All Site Settings / Site Features.
The feature that needs to be activated is "Office Sharepoint Server Standard Site features," as shown below.
I recently spent some time looking for this even though I knew where it was. The Slide Library is not specifically mentioned anywhere on the Site Features page. Hope this post saves someone a few minutes of frustration.