I had unfortunately picked up the (bad) habit of debugging my SSIS packages by using messagebox.show at design time. The SSIS data flow Script component does not allow debugging using this approach so I had to find an alternative. I discovered that it is possible to output parameters at design time using the FireInformation in the Script component. This puts the parameters in the progress window.
// Output parameters into the SSIS progress window
bool FireAgain = true;
this.ComponentMetaData.FireInformation(0, "DataFlowDebug", "The value of time is " + Row.ParamValueTime.ToString(), "", 0, ref FireAgain);
this.ComponentMetaData.FireInformation(0, "DataFlowDebug", "The value of environment is " + Row.ParamValueEnvironment.ToString(), "", 0, ref FireAgain);
I like this approach although I should probably output the parameters to one of the other console windows.