When Setting up Set Field, there are two Specify buttons that must be clicked. To get Set Field [Table::Field ; Expression], add set field to your script and click the first button (specify target field). Select Table::Field from the list of fields. Click OK. Now click the lower specify button (calculated result) and create the expression to the right of the semicolon (;). Do not try to type in the semicolon.
Assuming you are in scriptmaker...
See the two buttons on the lower right of the window (Specify Target Field, Calculated Result)
Click the top one to choose your field (<tablename>::Count Field), hit OK
Click the bottom one, calc window pops up, type "0", hit OK
Done. FMP inserts the semicolon for you.
... and Phil beats me to the post again...
Thank you both - it isn't intuative, but I will get better at this . . .