What you are asking is how to use fill flash on your E-M5. Go to page 65 of the full user manual -- you can easily download it on-line.
Make sure you set the maximum flash synch to 1/250 Sec in your camera menu. If you set the flash to fire all the time for fill flash, you should have no problem. Right now, it sounds like your flash is set to Auto Flash mode which will keep it from firing in the camera meter indicates enough light.
Thank you. your explanation is clear. The instruction in the manual are to
Display the super control panel or live control and highlight the flash mode.
My problem is that when I click menu I don't see the options of the flash mode that are in the manual....