Tell us more about what you are trying to do, and with what software you are trying to do it. Are you using Picard, the web interface to the MusicBrainz database, or something else? Are you trying to type new tag values into the table of metadata values in the lower part of the Picard user interface? Or are you trying to get metadata values downloaded from the MusicBrainz database, and those values have accented characters in the database?
If you are trying to type in accented characters, you should be able to type them in the same way you would type them in in a word processor. Do you know how to type in accented characters in your word processor? How do you type them in there? What happens when you do the same thing with whichever MusicBrainz tool you are using?