In MM-15, Text to Speech is explained on page 85 of the manual. I've never tried it, but it sounds simple enough. Apparently you would use the LOAD command in the text to speech editor to import your rtf file.
If that doesn't work, can you cut and paste your text into the program?
(The full manual is accessed through the Windows start menu. (Start/All Programs/Magix/Magix MM-15/Documentation/ Manual) If you are using a Mac, you're on your own.
just click "Effetcs" > "Audio" > "Text-to-speech"
Write or paste your text into the editor. Click on "test" and you can hear how it would sound.
Afterwards you can create an audio object and put it into the arranger.
Now you have the opportonity to edit your text-to-speech-object with plenty of effects. If you use the premium version you can also edit it with the "Elastic Audio" function.