Generally you use ActiveX to control Excel from LabVIEW. The best way to understand that would be to run 'Write Table to Excel" example that ships with labview. You can access that example by going to Help>>Search Example and then under "activeX".
Also for more flexible control of excel from labview you can also use the latest "Report Genaration Toolkit" for labview that allows you to interact not only with Excel but also MS Word.
http://digital.ni.com/worldwide/bwcontent.nsf/web/all/E899E563ABE94D1D86256A1800718B6D
Hope this would help.
A Rafiq
National Instruments