To address this, open Motion 4 from Final Cut Studio (2009) (located in /Applications/Final Cut Studio). After it opens completely, quit Motion 4. Final Cut Pro 7 should now open successfully.
Final Cut Pro X, Motion 5, Compressor 4: May be unable to open Final Cut Pro 7
You may see the following alert message when attempting to open Final Cut Pro 7; this can occur after installing Final Cut Pro X, Motion 5, or Compressor 4 on a system that already has Final Cut Studio (2009) installed.
"This version of Final Cut Pro is incompatible with Motion 5 which is installed in a directory that previously contained Motion 4. Please move Motion 5 to another directory and restart Final Cut Pro."