first get yourself a bittorrent client. you can find the standard client at bittorrent.com
then, register for dimeadozen.org. takes all of five minutes.
then, once registered, go to torrents > search, and enter "Dream Theater" (including "").
you'll get a list of shows, the miami show was at the top of the list when i searched it. click on the link, and it will take you to a page with the show info, setlist, encoding info, and region of the dvd (NTCS).
click on the link for the torrent itself, and it will download to your computer. double click on it, and it will load your bittorrent application. from there, it should start automatically, but if it doesn't, select the file in the menu, and press the play button at the top of the client. the whole 4.32GB show downloaded for me in under an hour. now, dimeadozen has a fairly strict ratio rule (ie, how much you download vs. how much you allow others to upload from you), so just make sure that you leave the file uploading for at least an 80% ratio (this can be set in preferences).
big note: this is completely legal. DT allows performances to be taped, video and audio, depending on the venue. if you want to keep it this way, never ever ever sell or buy bootlegs from anyone except the band. selling for profit is not only illegal, but is the quickest way to get DT to stop allowing people to record their shows, which would suck.
big note two: bittorrent can be used to illegally download copyrighted material. do so at your own peril. this can result in anything from having your internet turned off by your ISP, to prison terms and very very high fees. if you're not sure if something is legal or not, assume it's not and don't download it. if it's something like live shows, check the band's website, or forums like these, to check legality issues. almost every band will have a taping policy somewhere on their website.
enjoy the show!