A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
Right-click a Column & select Format Data Series from the contextual menu. The Format Data Series task pane will display what you're looking for. Note that the controls presented in the task pane will vary depending on the specify type & style of chart.
Since you give no indication of why you're having difficulty following those instructions I'm not sure what else to suggest. They appear to be written based on a Windows version of the program as the model but translate easily here. The main difference is that step 2 refers to the list of chart elements as being on the "Format" pane, whereas in Mac Excel it's on the Format contextual tab of the Ribbon.