When I try to delete images from my SD card, I get a message saying "Memory card is locked. Slide lock to "write position." Where is this lock?
The write-protect lock on an SD card is a very small plastic slider on the side of the card itself. It should be labeled as such, probably in fine print.
(File modified from original on Wikicommons by Afrank99; CC-BY-SA 2.0)
As you can see from the arrow in the picture, slide down to lock, or up to unlock.
Note that if the switch breaks off, that will leave the card in the locked state. If you can't find anything, that may be what happened.