You can use regedit to modify registry variables, and you need to look in HKEY_LOCAL_MACHINE\SOFTWARE\ORACLE for the forms 6i keys.
Create or modify the FORMS60_PATH key with the value
[tt]<existing path>; F:\abc\def\ghi; F:\abc\def\ghi\jkl[/tt]
Create or modify UI_ICON to point to the location of your icons.
If you do not know how to use regedit then BE VERY CAREFUL when using this util as the registry contains many system variables that can mess up your system if you change them.
An alternative way of entering registry variables is to create a .REG file. Create a text file containing the following lines:
[tt]REGEDIT4
[HKEY_LOCAL_MACHINE\SOFTWARE\ORACLE]
"FORMS60_PATH"="F:\\abc\\def\\ghi; F:\\abc\\def\\ghi\\jkl"
"UI_ICON"="F:\\abc\\def\\ghi"[/tt]
Call the file FORMS.REG. When you have saved it, simply double-click it from Windows Explorer and it will save those keys to the registry.
As a final note, the path Oracle forms will take in its search for other forms or called modules is as follows:
1) Current Working Directory
2) FORMS60_PATH
3) ORACLE_PATH (also in HKLM\SOFTWARE\ORACLE)
The search for icons is similar, except that UI_ICONS is used instead of FORMS60_PATH.