Slider in Applescript?

You need to specify the complete hierarchy of the item (I am guessing you have the various items named and connected in IB), for example:

tell window of theObject to set (rate of my_movie) to contents of slider "slider"
-- or --
set (rate of my_movie) to contents of slider "slider" of window "whatever"

By hierarchy, I mean the levels of objects that make up the various parts of the user interface. For example, the Finder describes a file item as file "x" of folder "y" of disk "z". An application may have a window, and that window may have tab groups, and one of the tab groups may contain a text field, and...

When you access an item, you need to fully describe what it is so that AppleScript can pass the message on to the appropriate object. If your slider is a part of another item (such as a window), you will need to include that item in the path to your object.

The messages in the build results will tell you the line where your script is failing - if everything is named and linked, my guess would be that the use of your application terms inside of the Quicktime tell statement is confusing things. Typically, to avoid terminology conflicts inside of tell statements, you should try to avoid putting anything into an application tell statement that isn't directly related to that application. Try breaking the item(s) out, for example:

<pre style="
font-family: Monaco, 'Courier New', Courier, monospace;
font-size: 10px;
margin: 0px;
padding: 5px;
border: 1px solid #000000;
width: 720px;
color: #000000;
background-color: #FFEE80;
overflow: auto;"
title="this text can be pasted into the Xcode AppleScript Editor">
on action theObject
tell window of theObject
set slider_value to contents of slider "slider"
end tell
set my_track to choose file
tell application "QuickTime Player"
close every window
open my_track
set my_movie to first document
set ts to time scale of my_movie
set current time of my_movie to 0 * ts
set rate of my_movie to slider_value
end tell
end action
</pre>