Before you can modify a formula, you must insure that the data file used to create the formula and the model file are both open in ChaosHunter.
To open a data file, select the ChaosHunter File Menu and then choose the open data file option or click on the file folder icon on the toolbar 
. By opening the data file you have access to different inputs that may be selected in the Formula Editor.
To open a model file, select the File Menu, open model option and browse to the model file you wish to edit or click on the disk drive icon with the arrow on top on the toolbar
. If you've just optimized the model in ChaosHunter it remains open in ChaosHunter. However, you might want to first save the model file before making any changes. Select the File Menu and choose Save Model before proceeding to the Formula Editor.