Excel spread sheets created on a PC show up as "read only" when opened in my iMac which has Excel. How do I correct this?

Select the spreadsheet in Finder and do a Get Info on it. Look at the Info window and make sure you have read and write permissions to the spreadsheet and that it is not locked.

Are you saying when you open the Info window on the file and try to change the permission it won;t let you? Do you get any messages? Is the account youare using an Admin account?

If you could post a screen shot of the Info window opend on the file in question it could help.

If the Mac is reading the Excel workbook directly from a Windows NTFS disk partition, be aware that without 3rd-party tools, OS X can only read files from NTFS, not write files.