Did you convert the video to one of the supported formats shown here:
What video and audio formats are supported?? - BlackBerryFAQ
What OS is your device? You need 4.5 to use the full 8 GB card.
You can get Desktop Manager with or without Roxio. Sounds like you installed a version without Roxio. I used Roxio to convert videos for my device and it worked nicely except it's a bit slow.