When the task was already created:

- Tap the task you wanna tag;
- At the right upper corner, there is a button with three dots (...). Tap it;
- A context menu will appear; tap the option "Edit Task";
- There are several options, including taging;
- Tap the disk icon to save the changes.

With a new tag:

- Tap the 'plus' (+) icon to add a new task;
- Write down the task's title
- Tap the 'tag' icon (I don't know how to describe it; it kind of looks like a little flag)
- Select the tag(s) you want to add to the task;

OR

- Add this symbol '#' when you are writing the title; it should appear a list with all the tags you've used in RTM.

I hope that helps, and sorry for my bad English.

carlos.schults covered all the bases! (Thanks!) When editing an existing task, if you don't have Tags as one of your default fields and don't already have a tag added to the task, you may need to tap the Add Field button at the top (the leftmost icon, to the left of Save) to add Tags before being able to add a tag.

Hope that helps!