If your document uses user-defined slur contour settings (i.e. not Finale's default settings), you need to manually enter these settings here.
If you haven't changed these settings - most users probably haven't -, skip this step.
Unfortunately the Perfect Layout plugin currently can't read these values automatically.
To get the settings open SmartShape->Slur Contour and write down the Height and Inset settings for all four span widths (short, mediumm, long, extra long).
Then run the plug-in and enter these values by clicking on the "Enter Slur Settings from SmartShape->Slur Contour
These values will automatically be saved in your plug-in .ini file and only need to be entered again when they change.
In most cases users don't have to change anything here.