You can Add Tags to Tasks/Notes (and notes) anywhere you see a task/note.
To add tags to a task from a Tree View or List View, click the [Edit] link in the 'Tags' area to show the tag edit field for that task. As you type into them tag edit field, existing tags will appear in a drop down list. To use an existing tag you can either keep typing the tag or use the arrow keys and enter key or mouse to select the tag you want from the existing list.
To add tags to a new task you are creating, enter them into the Tags field in to Edit Screen.
In the List View you can change tags for multiple tasks at a time. You can choose to append new tags to selected tasks or set tags for selected tasks (overwriting any existing tags).