Okay,
Select your chart. This will cause a contextual menu to appear along side
the existing tabs on the Ribbon.
The contextual menu is labeled Chart Tools. Within that are 3 contextual
tabs. Design, Layout and Format.
Select the Layout tab. This will display various groups on the ribbon
labeled, Current Selection, Insert, Labels, Axes, Background, Analysis and
properties.
Within the Labels group is a dropdown button labeled Axis Titles. Here you
can pick which axis to label and how.
Once the axis text box is displayed in the chart you can edit the text as
normal.
Cheers
Andy