1) Please don't type in all caps. It is considered shouting and some people may not even respond.
2) Nextel is different - do you have a BlackBerry Data Plan on your account? If not, you will have extremely limited use of your BlackBerry.
3) The Desktop Software (called Desktop Manager) is available from the RIM Support web site at BlackBerry
4) Check for themes (not a lot out there for the 7520) on the dedicated Ringtones, Themes and Wallpapers forum.