Can you tell us what version of Office for mac you are using?
Are you just wanting spelling support or do you need the aditional characters also?
Are you using Office 2004 or 2008?
Are you just wishing to use Greek language suport?
Word on a PC uses a thing called unicode to display Greek in Word whereas Mac does not natively support Greek language except for a limited use in Office 2008, So I am going to assume you are probably using 2004.
Coudl you download and install XGreek from here then restart your Mac and let me know if your documents open correctly http://www.macupdate.com/info.php/id/13444/xgreek
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To be honest it may be that you do have Office v.X installed, I that is the case you can download a trial version of Office 2008 from here http://mac.softpedia.com/get/Business/Microsoft-Office-2008.shtml once it is installed you should then be able to read your Excel files so yu can print them or copy them to a different file format such as PDF.
Let me know what happens.
Office 2008 (released Jan15th 2008) is the latest version available for Mac then 2004 (released May 11th 2004) then Office X (which was released November 19th 2001)
The office X program does not support Unicode in any application so it is incapable of supporting the Greek lanuage unlike its PC cousin which supports it without any issues.
To put it simply, you will need to install a minimum of Office 2004 to suport Greek in Excel, However Office 2008 would be better.
Let me know if you need more info or clarification.
I think i see what you are asking, To get the Greek support you need to use Unicode, However Office X does not support unicode in the Excel application which is why you can not open the Excel file from your PC.
Only an aplication that supports unicode will allow for the opening of you Greek files which is why you need to use a newer version of office to allow the Excel files to be opened.
Does that make it as clear as mud?
One thing yu may wish to try if you dont mind another software package rather than Microsoft office is Open Office http://www.openoffice.org/
Open Office is a free office suite for all operating systems which can open Microsoft format documents such as Excel and it also suports unicode so it should support your Greek language needs and files.
Take a look and let me know what you think.
Sorry to say but a sutable spreadsheet app is not natively on your mac which is why I would reccomend you try Open Office as it should be able to support your current pc Excel files on your mac and support the Unicode you need for future projects.
Plus open Office is a totally free application suite (you never have to pay a single pennyto use it or download it), Take a look at their website here and check out the I want to learn more section http://www.openoffice.org/
Let me know what you think.