Paratext 7.5 doesn’t look for the FieldWorks 9 installation.
I experimented and it looks like you can create the registry entry that is needed and then get Paratext 7.5 to find FieldWorks.
Assuming you used the standard locations, you would need to run regedit and then create this key:
HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\SIL\FieldWorks\8
Then add a RootCodeDir string value set to:
C:\Program Files (x86)\SIL\FieldWorks 9\
I did my testing using the 32-bit version of FieldWorks 9 - if you used the 64-bit version, remove the " (x86)" value after “Program Files” for the RootCodeDir value. I’m not sure if Paratext 7 will work with the 64-bit version.
John+Wickberg
Paratext Support