I do not think you can do all of the steps with Seahorse and the seahorse plugins do not seem to be maintained beyond 11.04.
Create a gpg key
Start seahorse , under the main menu New -> Create a new GPG key
Fill in the subsequent information and password
2) Download the PGP key from Truecrypt You can do this with any browser, save the file where you wish. For this tutorial I will use Downloads in your home directory.
The file is called "TrueCrypt-Foundation-Public-Key.asc"
3) With seahorse, import and sign the key
In the seahorse menu -> File -> Import
Select the truecrypt key ( "TrueCrypt-Foundation-Public-Key.asc" )
Now sign the key as trusted
Under the "other" tab, select the truecrypt key and sign it as trusted
4) Download the truecrypt PGP signature from here, save it in Downloads
5) As far as I know, you can only verify the signature from the command line
~/Desktop$ gpg truecrypt-7.1a-linux-x86.tar.gz.sig --verify truecrypt-6.2a-ubuntu-x86.tar.gz
gpg: Good signature from "TrueCrypt Foundation "
See also http://mindlev.wordpress.com/2009/09/07/verifying-the-truecrypt-installer-package-on-ubuntu/