There are 3 ways to dynamically modify a MaxScript UI.
One way to to create all the UI elements you need in order to disable and enable them as needed. (This is the most common method)
A similar method is to use their visible property to turn them off and on.
The most complex way is to generate a maxscript on the fly and use the execute command to create the UI elements.
This script has an example of each method: Martinez_DynamicUI.ms
