Open the property dialog for the Execute Process task. Go the Expressions page (third option in the list on the left). Click the ellipsis (...) button next to the line that says "Expressions". In the resulting dialog, select Arguments from the drop-down list under Property, and the click the ellipsis again under Expression to bring up the expression editor. Drag and drop your variable from the list in the upper left corner to the text box labeled Expression towards the bottom of the dialog. Click Evaluate Expression and you should see the value of your variable. Then click OK until you are back to the control flow. The expression is now setting the Agruments property dynamically with whatever value you put into the variable.