Just to expand a little, you can't open Excel files in Monarch Standard, only in Monarch Pro, but yes, since v5 Pro this has been possible. To open an Excel file, run Monarch, then select Open Database (not Open Report), and browse to the file you want. You'll be presented with a dialog box allowing you to choose the worksheet or named range in Excel which contains the data you want.
Monarch assumes that the data in Excel is tabular - that is, each row is one record, and each column is one field. If your Excel data contains data where a record is described over several rows, then you'll need to treat the data like a report instead. If you need to do this, you will have to save the Excel file as one of TXT, PDF, XPS in order to be able to open it in Monarch as a Report.
Hope this helps,