// using Excel = Microsoft.Office.Interop.Excel;
// oDoc - ссылка на деталь с внедренным экселем
object exlWorkbookTemp;
Excel.Workbook exlWorkbook;
Inventor.ReferencedOLEFileDescriptor referencedOLEF;
referencedOLEF = oDoc.ReferencedOLEFileDescriptors.ItemByName["Name"];
referencedOLEF.Activate(Inventor.OLEVerbEnum.kHideOLEVerb, out exlWorkbookTemp); //kHideOLEVerb kShowOLEVerb
exlWorkbook = (Excel.Workbook)exlWorkbookTemp;
Excel.Worksheet exlWorksheet;
exlWorksheet = (Excel.Worksheet)exlWorkbook.Sheets["SheetName"];