What can I do?

[kalendor]

I try to install v-drift from
http://www.playdeb.net/install/vdrift
then begin to download and install but, suddenly appears this message
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There was a problem

W: Error de GPG: http://ppa.launchpad.net lucid Release Las firmas siguientes no se pudieron verificar porque su llave pública no está disponible: NO_PUBKEY EBA2799DEF21CE39
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What can I do??

Thanks