The default forms editor in Sitefinity 9.0 is MVC only so if your widget is a Web Forms widget it will not show in the forms toolbox. In version 8.2 the forms editor only defaulted to MVC only if you had installed Sitefinity Editor.
If your forms widget is indeed WebForms you can change to Web Forms on the Create form dialog. See an example here: http://screencast.com/t/NOHpOSrUoUL2
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Sitefinity CMS Ideas&Feedback Portal
and vote to affect the priority of the items