When placeing the next dim, don't place it. go up to modify prefix and clear the box's. this should clear the default
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.