Ubuntu, public key is not available

Question:

Recipe:

1. Go to http://keyserver.ubuntu.com:11371/ and search for
   0xnumber_in_the_error_message
   The above means you will search for the number which was given at the end of the error message, preceded by 0x (to tell that it is hexadecimal).
   
   
2. Follow the links to the page showing a public key block like this:
   -----BEGIN PGP PUBLIC KEY BLOCK-----
   Version: SKS 1.0.10
   
   mQGiBENO2vMRBADarzDwnL0/3t0Nbvh93j71ylG/sVTqfcSJwN6b7vx5aEiaBkUDrIJUR7sh
   some code here...
   and some more code here...
   and so on...
   ToFmNOwyeISw5Hmv9dXLeMgsAKCDEg83AaqvW9891hfaRpVIl+wfBQ==
   =vLhl
   -----END PGP PUBLIC KEY BLOCK-----
   
   
   
3. Select the block from
   -----BEGIN...-----
   until
   -----END...-----
   including these 2 lines
4. Type CTRL-C
5. Open gedit
6. Type CTRL-V
7. Save this as a file named
   some-app-public.key
8. Go to menu
   System > Administration > Software sources
9. Go to tab
   Authentication
10. Click the button
    import key file
11. Select the file you did just save
12. Ok, Close