password protect excel file?

[Dawg751]

I have an excel file which was sent to me that I need to place password protection on so it can place it accessable from within a web site. The instruction I have found say to open "Prepare" from the Select Office button, then select "Encrypt Document" to add the password. My problem is that the "Encrypt Document" selection is grayed out and inaccessable. Anyone know what is causing the gray out. I thought perhaps it is because I did not "author" the document.
Looking for help!!
Thanks