I have a computer monitor I would like to use on my security system. The monitor has a hd15 connection and my DVR has a bnc connection. when I search on line the cables have 1 hd15 to 5 bnc connectors. How do I connect the two?

You don't, not with that cable - the five BNCs are for the video component signals: red, green, blue, horizontal sync, and vertical sync. Your DVR's BNC is outputting a full composite signal.

You need a composite-to-VGA converter. I've used some before, can't remember what brand they were though. This one is kinda overkill, but it's along the lines of what you need: http://www.nti1.ca/vga-converter.html

Another example:
http://leosecuritycctv.com/shop/item.asp?itemid=638

And a whole series of examples:
http://www.nextag.com/video-to-vga-converter/search-html