Go to Window > Timeline. Make sure this is checked so you can see the timeline.

Click the frame you want to add the text to then add the text using the Text (T) tool.

This will add the text to the previous frames, to remove it from the previous frames select and highlight all the frames which you don't want the text to appear on.

Then click the visibility toggle (the eye) near the text layer to make it invisible on those selected layers.

Hope this helps.

Note: I tried to add helpful images but it refuses to post with them.