Adding Trend Lines to Excel Charts

My data/chart is a simple 'line' chart. Clicking on one of the data points in the plot "illuminates" the rest of the data, but opening the Excel 'Chart' menu still results in a grayed-out 'Add Trend Line' label, so there is no way to cause the line to be created and inserted in the chart. I've tried to get a trend line created with other sets of data, and with xy scatter charts, and get the same result ("Add Trend Line" is grayed out in the "Chart" menu).

I've tried getting help through the Mactopia Excel forum, but without success.

Any suggestions would be appreciated.

My Excel version is 12.1.3 (080930) with the 12.1.4 update; my Mac OS X is 10.5.5.