Update to the latest version posted to the link above:
http://sourceforge.net/projects/msnpl...dium/files/

OK, I tried this script under Leopard and it fails to show the dialog box. I hada look at the script, and managed to fix it. I hope this doesn't get mangled in the comment:
1. Right-click on the pickup.AdiumScript file and choose Show Package Contents
if the script is installed, it is located in ~/Library/Application Support/Adium 2.0/Scripts
2. Open up the Contents folder, followed by the Resources folder, and double-click the pickup.scpt file; this should open up the Script Editor.
3. Change the lines that read:

display dialog pickupplus buttons {"Forget It", "Try Again", "Use"} default button "Use"
if (button returned of result) is "Use" then
return pickup
else
if (button returned of result) is "Forget It" then
return ""
else
getpickup()
end if
end if

to read:

tell application "Adium"
display dialog pickupplus buttons {"Forget It", "Try Again", "Use"} default button "Use" with title "Pickup Line"
if (button returned of result) is "Use" then
return pickup
else
if (button returned of result) is "Forget It" then
return ""
else
my getpickup()
end if
end if
end tell

Basically, you've added the 'tell application "Adium"... end tell' block around it, and added 'my ' in front of the 'getpickup()'
Click Compile, and Save it from the File menu. Done.

Martin