The list of available dimensions can be found in the menu bar on the left.
Select your chosen dimension and drop it to the right, in the designated area.
Once you've added a dimension, the list of dimensions to the left will automatically update. In addition to the first dimension selected, you may use all other dimensions which are still displayed for your analysis.
You can collapse all dimension categories for a more streamlined display.
Tip: When searching with the menu bar collapsed, the dimensions contained in the collapsed categories will not show. To avoid this, be sure to expand the menu before searching.