Take a look at the ADD operation on your EXCEL_SHEETS signature. When I imported the signatures, the classification of the RESULT_ parameter came in as "?UNKNOWN?". I changed this to EXCEL_WORKSHEET. After that, the following code worked:

newinstance "EXCEL_APPLICATION", $excel$

$excel$->set_visible(1)

$excel$->get_workbooks($workbooks$) ; get the application's collection of workbooks

$workbooks$->add($workbook$, -) ; add a new workbook to it

$workbook$->get_worksheets($worksheets$) ; get the new workbook's collection of sheets

$worksheets$->add($worksheet$, -, -, -, -) ; add a new sheet to it

The variables are all defined as public handles.