Office for Mac 2011 - can't open old files

I am currently on Mac OS X v10.6.8. I recently installed Office for Mac 2011. I cannot open some of my old files. I think the ones that use the old file names, e.g. .doc and .xls, rather than .docx and .xlsx.

Can anyone explain how I can open these files in my Office for Mac 2011?

Many thanks!