Hey Diana,
First of all, what version of DIAdem are you using?
Secondly, right-click on the file (from the Navigator view) and select "Open With". Then select "Excel Files". Does this launch the Excel Import Wizard?
Finally, if you are not able to launch the Excel Import Wizard with this method, please attach the excel file (or an arbitrary Excel file exhibiting the behavior).
Thanks,
Andy McRorie
NI R&D