You can use WScript.Arguments to access the arguments passed to your script.
Calling the script:
cscript.exe test.vbs "C:\temp\"
Inside your script:
Set File = FSO.OpenTextFile(WScript.Arguments(0) &"\test.txt", 2, True)
Don't forget to check if there actually has been an argument passed to your script. You can do so by checking the Count property:
if WScript.Arguments.Count = 0 then
WScript.Echo "Missing parameters"
end if