The personal fit models are stored in a specific directory as indicated in the Administration Console's Maths and Statistics section under Model Settings. Ensure that the User fit Model File is pointing to a valid directory and that the directory contains two .xml files.

Open up the Formula Editor and right-click on Fit Models. Select "Add Category" and give the category a name. Right-click on the new category and select "Add Fit Model". In the resulting dialog box, give the model and name and type in the formula you wish to use. Choose OK to close the dialog box and then OK to close the Formula Editor.

Open up the Fit Designer and select the Model tab. Scroll down to the bottom of the model list and ensure that your new category and model tree is listed.