The APC Back-UPS ES 550 is a consumer-grade UPS: the software it comes with does not work in Linux. Instead, you need to use NUT or apcupsd. Google will turn up evidence that both work.
The original NUT instructions that this article is based on came from [http://www.mscs.dal.ca/~selinger/ups/backups.html here] and [http://billauer.co.il/blog/2010/01/fedora-mustek-powermust-600-ups-nut-howto/ here]. The former, by Peter Selinger, are closest to this case, but a lot of the work he had to do is now done for you out of the box. The latter are for a different UPS, but are also fresher.
First, do this: