There are two ways to use Excel within Budgyt, as supporting documentation, or as a linked file.
Supporting Documentation - to attach an Excel file purely for informational purposes, go to the Data Sheet, click the edit tool in the appropriate Department and Category, and use the document icon at the far right to attach your file to the sub category row.
Linked Files - to use an Excel file as an integral part of your budget, go to Data/Formulas > Manage Excel Links, click the Template button to download the template, and insert this a tab into your file. This is the tab that Budgyt will read. You'll reference the relevant data in your other tabs from this sheet. Then follow the instructions in the template, hit Add New in Budgyt, and choose your new file.
This video will give a more complete overview of how to link files: