If installing on FrameMaker 8 or 9 you must edit the maker.ini file to disable the existing DITA support.
DITA-FMx cannot coexist with the default FM8/9 DITA support. An error message will display to remind you of this if you neglect to modify the maker.ini file.
Open the maker.ini file and locate the APIClients section. Look for lines that start with ditafm, ditafm_app, ditabook (FM8 only), and possibly ditaOpenToolkit (if you’ve installed the DITA OpenToolkit connector). Delete or comment out these lines (by adding a semicolon at the start of the line).
You may now proceed to Run the Installer Application.