There is a bug for it here:
https://bugs.kde.org/show_bug.cgi?id=182052
And here is the applet:
http://www.kde-look.org/content/show.php/Run+Command?content=91495
Looks like it's been updated recently. I'll check it out.

On Wed, May 6, 2009 at 11:54 PM, Dotan Cohen <dotancohen@...> wrote:
>> 2. The "Run Command" plasma applet should be fixed up a little to
>> start working better as a panel applet.
>>
>
> Has this issue been resolved for you? I asked for more info earlier,
> but did not hear back from you.
>
> --
> Dotan Cohen
>
> http://what-is-what.com
> http://gibberish.co.il
>