The SmartDimension CommandBar should appear when you click on Smart Dimension tool. If that's not happening, it might be in a collapsed panel on the side of the screen or it may have been moved off of the screen. You might try going to Solid Edge Options>Helpers>Command User Interface, and switching to either Use Horizontal Toolbar Form or Use Vertical Docking Window Form, and see if that resolves the problem. If not, there is a registry key you can use to clear the interface that will set it back to default. Or, you could use one of the pre-set Themes.
If you change the theme to Some Assistance or Maximum Assistance, it will rearrange your interface. It may be a good idea so save your current interface as a theme in case you want to return to it.