All ways I have found on your website don't go!
Nontheless: try this:
{source}
[[script type="text/javascript" language="javascript" src="http://www.donnerwetter.d
e/wetter/net/boxregio.mv?typ=2&plz=88081&color_bg=FFFF6 6&color_hi=FFCC00&color_txt=&
width=100"]][[/script]]
[[noscript]]
JavaScript ist deaktiviert - leider sehen Sie hier kein aktuelles Wetter. [[a hre
f="http://www.donnerwetter.de" target="_blank"]]www.Donnerwetter.de[[/a]]
[[/noscript]]
{/source}
Then, if it still doesn't work, what do you get for code in the final html output?
Do you get the script but with differences?
Do you get a Sourcerer! comment telling you that the script is stripped for some reason?
Does the script come out correctly, but you get JavaScript errors? And what are the errors?
...