First, if you haven't already, set and access key under the "Preferences" -> "Create/Change Access Key"
Then, go to the bb-usa.host22.com, and log in using your Buzzerbeater username and your access key (not your buzzerbeater password).
Underneath the USA banner with the Eagle, there's a light blue strip with one option being "Team Tools"
Hover your mouse over that, and a drop down menu appears and choose the "Training Planner"
When you click that, it should bring you to a list of your players, followed by a series of drop down menus where you can change his skill value (choose "Synchronize Roster" if things don't match exactly, or you've added new players).
As you choose new skill values, two things are updated on the left. One is a column called "Est". That's the estimated salary of the player with those new skills. To the right of that is an unlabeled column. That is the potential cap value for the skills you entered, calculated using Josef Ka's current coefficients. To the right of that is a column which contains a value like "18-20". That represents the potential cap given the player's potential.
Hope that's helpful! Lemme know if anything goes wrong when you try playing with it.