Repro steps:

Create c:\p4\installer\project\foo.sb5
Create c:\p4\installer\bar.sbi
Open foo.sb5
Add an #include script... line
Set the path to ..\bar.sbi
-or-
Set the path to [SB5_PROJECT]\..\bar.sbi

Expected results:

bar.sbi can be found

Actual results:

SetupBuilder tries to find
C:\Program Files\Lindersoft\SetupBuilder 5 Developer\Include\..\bar.sbi
-or-
C:\Program Files\Lindersoft\SetupBuilder 5 Developer\Include\[SB5_PROJECT]\..\bar.sbi

Notes:

SetupBuilder is smart enough to recognize a full path like c:\p4\installer\bar.sbi. But not everyone maps their source control enlistment to c:\p4, so for them the script is broken.