There are a couple of things very specific to getting movies to play on land.
1) You must have permission/media rights to change the media texture on the land.
2) The only way to install the texture into the media tab, is by clicking on the blank square in About Land/Media Tab, and choose the exact same texture that you use on the screen(s) from your inventory (don't try dragging and dropping it, it won't work).
3) The texture must be full mod.
4) The texture must be larger than the movie resolution (rounded up to the set sizes i.e. 512x512, 512x1024 etc).
5) Instert the full url of the movie i.e. (
http://www.<mywebsite>.com/mymovie.mov ) and make sure it points directly at the movie, do not use a html file to show the movie, SL cannot do that.
6) Apply the same texture to a plywood prim (preferably on one face only) and make the screen the same size as the movie resolution i.e. 320x240 = 3.200x2.400, click the 'All Bright' function in the texture tab, to gain a better quality picture.
7) You should now see the movie player controls, click play.

Edit the screen, in Texture tab, click the align button (you can only do this if and when there is a movie stream live). This will make the movie squarely fit the screen. If you want to make the screen bigger/smaller, check, 'stretch texture' and use the corner grey boxes of the screen to maintain aspect ratio.
If the screen goes black but you don't see a movie, it could be a couple of things.
a) The movie is 'download and watch' format (change it to Progressive download).
b) You have not set your preferences correctly in your client (IM me ingame and I'll drop you a notecard).
c) Your system isn't powerful enough to view movies (Don't IM me ingame).