[Lizard]

1) Save the Excel file as CSV
2) Drag the CSV file onto the OmniPlan app icon (or choose "Import" from OmniPlan's File menu).
3) You should get a dialog asking you how to match up columns from the CSV file to OmniPlan's data columns.

Depending on exactly how the data is in the Excel file, you might want to copy just the task names and properties, without the gantt chart part, to a new file. That will probably make the import process go more smoothly.