Starting with version 8, Prism presents some results on multiple tabs. Multiple comparison results are always on a separate results tab than the main ANOVA results. Click on the Multiple Comparisons tab to see those results.
When the titles are short, Prism will report the multiple comparison results using those titles, for example "Treated - Control". If the titles are on two or more lines or are long, Prism will report for example "Column B - Column A". How long is long? You set the threshold (for one line titles) in the Preferences dialog.