Jerry,
To add series names to the legend, first click on your chart to activate it.
Then, go to the standard toolbar and hit Chart -> Source Data -> Series Tab.
In the Series dialog box you should see a field called "Name". You can add
a name for each of your two series in this field. To move between the two
series to individually add a name, click on "Series1" or "Series2" on the
left side of the dialog box under the "Series" option.
Another way to add series names is to add titles above each of your columns
before building your chart. For example, using the data below start column 1
in cell A2, column 2 in cell B1, and column 3 in cell C1.
Title1 Title2
a 2 3
b 3 4
c 2 3
After the data is entered into the spreadsheet, go to cell A1, activate the
Chart Wizard, and go through the prompts. The Chart Wizard should
automatically assign "Title1" to series 1 and "Title2" to series 2.