My videos are dark but my still photos look great with flash
With most SLR cameras the video recording is done with the same exposure mode that the still images are done with. This can be problematic if you are using a higher power studio type flash unit with manual exposure. The flash is far brighter than the built in modeling light on those type units as the modeling light is intended for approximating the direction and quality but not the intensity of the flash. It is common to see manual exposure settings of ISO 100 with an aperture of 11 to 16. To have enough continuous light for video at that exposure it would need to be as bright as the afternoon sun. The bottom line is balance, you need to find a compromise exposure that will work for both. Try these steps;
First, start with the continuous light at the brightest you can use and adjust the exposure (the ISO and the aperture) of the camera until the image looks correct on the live view on the back of the camera.
Next, take a test still image at that same exposure with flash set to the lowest setting
If it is too dark increase the power on the flash unit and do another test still photo and continue that process until the test looks good.
If it is too washed out at the lowest power setting then you need to add more continuous light so you can lower the exposure or possible get a lower power flash
An alternative is to use the continuous light for both still and video or use Program mode with the built in TTL flash on the camera