How to use the public key in Ubuntu 12.04

Hi, all friends!
   I want to install graph_tool in Ubuntu 12.04.
   I add the source to my sources.list and update the source.
   It replys "W: GPG error: http://downloads.skewed.de oneiric Release: The
following signatures couldn't be verified because the public key is
not available: NO_PUBKEY 697EC21F15F95514"
   I don't know how to add the public key.
   I tried to save
F36FE35D<http://pgp.skewed.de:11371/pks/lookup?op=get&search=0x04DC461EF36FE35D&gt;in
keyfile and "sudo apt-key add keyfile" .
   But it says"gpg: no valid OpenPGP data found."
   What is this problem? Thank you for your suggestion.

2012/6/26 <graph-tool-request(a)skewed.de>

attachment.html (12.6 KB)

This is because the keyserver pgp.skewed.de is not working very reliably
at the moment, and you probably downloaded a 404 page or something
similar. I've just restarted the server, and it should be working
now. But if you have difficulties again, you can try any other key server, e.g.,

     http://pgp.mit.edu:11371/pks/lookup?op=get&search=0x04DC461EF36FE35D

Cheers,
Tiago