The exact location in the Registry will vary slightly depending on which version you have installed. Here is one example:
The real question is why are you looking for it in the Registry? This entry is for the Builder and Compiler only. Runtime entries will be found in default.env, assuming you are using a web deployed version (i.e. 9.0.2 or newer).
If you are using forms 10g go to *"ORACLE_HOME\forms\server\default.env"* and set the path at parameter FORMS_PATH
If you are using forms 11g go to *"ORACLE_HOME\user_projects\domains\"Your Domain"\config\fmwconfig\servers\"Your Server"\applications\formsapp_11.1.2\config\default.env"* and set the path at parameter FORMS_PATH