In FineReader PDF 15 there is no button to switch to the Form Editor.
The Form Editor has been implemented in Release 5 build 126.96.36.19964 of FineReader PDF 15. If an earlier version of the program is installed, the Form Editor will be unavailable.
In order to have access to the Form Editor, please update your copy of the program as described in the article: FineReader PDF 15 update.
After FineReader PDF 15 has been updated, please start the program, open any document in the PDF Editor and check if the Form Editor is available.