Not sure is you are using Appleworks in OS X, but if so, try this.
Open Appleworks, then go to File>Open. When the navagation box comes up, change the "File Format" selection from Appleworks to All Available.
Or, you can scroll down to select Excel spreadsheets.
I suspect this will help--why Appleworks is the default (with no apparent way to change that) makes no sense to me.