Go to Start > All Programs > Open Office and click on the relevant program. (e.g. Writer is the equivalent of Microsoft Word, Calc is the equivalent of Excel, etc).
Once the program has opened, go to File > Open, navigate to the location of the relevant file and double-click on it.
OR . . .
Go to the location of a file, right-click on it, select 'Open With' and choose the relevant option.