question

There's a couple of different ways you can accomplish this. One way that you can do it is to create a GUI button that changes the "enabled" variable in a source activity, using this code in the OnPress node (Flow1):

treenode mysource1 = getactivity("ProcessFlow", "Source1");
int enableval = getvarnum(mysource1, "enabled");
setvarnum(mysource1, "enabled", !enableval);
function_s(mysource1, "updateIcon");
repaintall();

The downside is that this method requires that you reset the model each time you press the button.

If you want to be able to click the button during the model run, you could create a button which will redirect tokens to an alternate branch on a Decide activity using a Global Variable, with this code (Flow2):

treenode globalvar = Model.find("Tools/GlobalVariables/Variable1");
globalvar.subnodes[3].value = !globalvar.subnodes[3].value;

You can reconfigure any of these two methods to fit your needs. Here's an example model:

GUIEnableProcessFlow.fsm