Is the file in your main blockland add-ons folder?
He just (basically) said no to that, he said it's in My Documents.
This is a problem, as the exec command works in relation to your BLOCKLAND folder.
In other words: the reccomendation of doing
exec("add-ons/Script_lul.cs");
meant that the .cs file needed to be in your BLOCKLAND Add-Ons folder.