Button does not have a label tag in its html structure. The text/label of a button is in button tag itself. So you need to specify css for the button element,
Open properties of button in mendix and add text-align: left; to style under common properties in Appearance tab.
If you want to create a css class then it would be like this.
.button-label-left {text-align: left;}
P.S: Increase width of button to see difference between left align and center align. Normally button size/width is according to its label so there is no visible difference between left align and center align. For testing you may set a higher width and then set align to left, center and right to see difference.