Hi Nancy,
To reference a cell on a different sheet or table, when you come to the point in your equatino where you need the address of the remote cell, just navigate to the remote cell and click on it, then continue typing or hit Return if that is the last item in the equation.
Of course you can also just enter the remote address manually. The syntax is Sheetname :: Tablename :: Cell.
If the table name is unique in the document, it is not necessary to use the sheet name.
About the names of things in Numbers - a little different than in Excel.
The entire document is referred to by Apple as a Spreadsheet, just to be consistent with the industry. I believe you called it a Workbook, an Excel term. Most folks here simply call it a Numbers Document.
What you called a Worksheet is a "Sheet" in Numbers.
Within a Sheet you can have multiple Tables, Graphics, Text Boxes, etc.
In Numbers, Tables are treated like Named Ranges in an Excel Worksheet. They should be kept to just the size needed for the data, and it's best to isolate separate collections of data in separate tables.
Regards,
Jerry