You need to have the seahorse-plugins package installed. It is already the case under Tails. Under Debian or Ubuntu, if you're not sure or want to install it, you can issue the following commands:

sudo apt-get update
sudo apt-get install seahorse-plugins

The development of seahorse-plugins has stopped. It hasn't been ported to Gnome 3. It is no more available on Debian starting from Wheezy and Ubuntu starting from version 11.10, Oneiric Ocelot. If you can't install it please try verifying the ISO using the command line.

If you are using Tails, you already have the signing key. Otherwise, first download Tails signing key:

Tails signing key

Your browser should propose you to open it with "Import Key". Choose this action. It will add Tails signing key to your keyring, the collection of OpenPGP keys you already imported:

What should Iceweasel do with this file? Open
with: Import Key (default)

You will get notified will the following message:

Key Imported. Imported a key for Tails
developers (signing key) <tails@boum.org>

Now, download the cryptographic signature corresponding to the ISO image you want to verify:

Tails 0.23 signature

Your browser should propose you to open it with "Verify Signature". Choose this action to start the cryptographic verification:

What should Iceweasel do with this file?
Open with: Verify Signature (default)

Browse your files to select the Tails ISO image you want to verify. Then, the verification will start. It can take several minutes:

Verifying

If the ISO image is correct you will get a notification telling you that the signature is good:

Goog Signature

If the ISO image is not correct you will get a notification telling you that the signature is bad:

Bad Signature: Bad or forged signature.