When a Dynamics GP form is modified, you need to 'set security' to the modified form before users can see the form. Note that this is not the case for forms that have only VBA behind them, only the modified forms.
To set security to a form, go to the Administration home page. Click on Alternate/Modified Forms and Reports.
Normally, we'd set security to DEFAULT USER. That will generally affect all users. So, if the user does not have the modified form loaded on their PC they'll immediately get errors. So... load the mods on the clients first.
Below, we create a special Modified Form ID called DEV, and in the screen shot below that we'll set the 'sa' user to look at DEV. That way only one user is affected.

