If you have the Report Generation Toolkit for MS Office, you can use the Excel Set Cell Alignment VI.
If you don't have the toolkit, you would use Range Vertical and Horizontal Allignment activex property.
Attahced example, in LabVIEW 7.0, is one of the ways you would use this proeprty. There are more ways to get the reference to range object.
Zvezdana S.