The problem is syntactical. In BrightScript, brackets [] designate a roArray while braces {} designate a roAssociativeArray. ShowVideoScreen() expects a roAssociatveArray as an argument, but you're trying to give it a roArray. It's also important to note that when you define multiple key/value pairs on the same line of a roAssociativeArray definition, they must be separated by commas. This is not the case if they are on separate lines.
Incorrect:
[Stream: url: "http://ia301529.us.archive.org/0/items/wonder_bread/wonder_bread_512kb.mp4" StreamFormat: "mp4"]
Correct (multiple attributes on one line with commas):
{Stream: {url: "http://ia301529.us.archive.org/0/items/wonder_bread/wonder_bread_512kb.mp4"}, StreamFormat: "mp4"}
Also correct (attributes on individual lines without commas):
{Stream: {url: "http://ia301529.us.archive.org/0/items/wonder_bread/wonder_bread_512kb.mp4"}
StreamFormat: "mp4"}