Open from main menu Explorer (Fig. 1 [1]). In the explorer, if you have Treelist view, expand following branches: Schemas -> [Your Schema Name] -> Triggers. For Tabbed navigation just switch to Triggers tab.
If you click on trigger, in the viewer should appears properties with Remarks row (which is comment for trigger) (Fig. 1 [2]).

Fig. 1. Trigger comments in Explorer

To edit this comment right click on table in the explorer and select Alter Table (Fig. 1 [3]). At the bottom of new opened window is field for comment (Fig. 2).

Fig. 2. Trigger comments in Explorer

After clicking Show Script button, generated script will appear. Click Execute button to save changes in database.

Fig. 3. Generated script by Toad