Hello,

in the tasks panel, there's the option "Titles and credits".  Go to the position where you want to add some text. Click it, and then choose "Title on the selected clip". Enter your text and choose "Add title".
In the Timeline, you can set the exact time when in the movie the text should occur.

I don't know, though, whether this is exactly what you wanted to do.

Greetings,
P. Di Stolfo

Guessing what you want...

Do you mean a Text Overlay which gives the appearance of floating over
your video or slide show?

If that's what you mean, go to timeline view, click on the + to expand
so you see the title overlay track. Select the appropriate type of
text and type in your text. Once it is on the overlay track you can
slide it around to more accurately position it. It is curious that
Microsoft placed the overlay track at the bottom below the main video
track, professional video editors put it at the top where it belongs,
since it is suppose to OVERLAY the underlying video. This is
accomplished by making the track an alpha channel meaning it's
background other than text allows what is below to bleed through. Kind
of confusing to visualize when Microsoft did it backwards. ;-)

On the other hand if you want to insert X number of frames that simply
stands by itself and appears for N seconds and NOT appear over the
video then make what you want in some graphic program then save as a
supported graphic file type, then import into Movie Maker and insert
it into the position you want and it will become part of the video.

This seems to be another missing common feature in Microsoft's movie
maker that's called a media generator from which you can create
patterns and background directly from within the editor which makes
things much simpler.