File path error in Applescript/TextExpander

Try sending the Activate command outside the System Event tell, that would be the standard method.

tell application "Canon MX510 series" to activate
my testAppRunning()
tell application "System Events"    
    key code 20 using command down
end tell

on testAppRunning()
    set test to 0
    repeat while test = 0
        log test
        tell application "System Events" to set test to count (every process whose name is "Canon MX510 series")
        delay 2
        --/*we do this even if active because it doesn't naturally come to front*/
        try
            tell application "Canon MX510 series" to activate
        end try
        if (test > 0) then exit repeat
    end repeat
end testAppRunning

After few different attempts and hacks I found good solution to solve my problem. First - implement the script. Then open the Automator.app and create new application. Then combine Get Specified Finder Items action and Run AppleScript. So now open -a my_automator_app /path/to/file will pass /path/to/file/ to my script. And that's cool.

P. S. in the argv will be stored the alias to the file, not the posix file path. To convert it from alias to posix you should use

set aliasPath to item 1 of item 1 of argv
set filePath to (the POSIX path of aliasPath)

Oh and don't forget that you might want to get the quoted form of filePath in case it contains some special characters (like space or ~).

You can get more information about alias/posix paths here