In the regular Excel window (not the Power Pivot window) if you go to the File tab in the ribbon, you should see Save to SharePoint under Save & Send in Excel 2010 or under Save As in Excel 2013.
Alternately, you could go into the SharePoint library, aupload a new document, browse to its location, and upload the file from SharePoint instead of saving to SharePoint from Excel.
Thanks i guess uploading from moss would work of course
but i cant understand why the save as to sharepoint is not visible under the file -> save as menu...
Please refer to the following similar thread about this issue:
In addition, publishing PowerPivot workbooks to SharePoint requires that you have sufficient permissions, software, and knowledge of how to prepare your workbook for publication. For more information, please see:
TechNet Community Support