OK I need to edit my answer. Once you get all 26, go to the Ruin Maniac just south of Veilstone City. The little room he was in before you had all 26 will be a huge tunnel, and at the end you will find an exit. Go in to find the final two Unown, ? and !. Hope that helps.
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The tunnel on top the ruins is accessible.The tunnel is where you catch the ? and ! Unown.
After collecting all 26 Unown, talk to the scientist and he will let you print out your Unown shapes.
In Diamond/Pearl/Platinum, after you have caught all the Unown the Maniac Tunnel on Route 214 will be fully accessible, now you can catch the "?" and "!" Unown.
they spell the alphabet and maybe you get a rare Pokemon
Each letter of Unown has its own room, which means that you cant catch all types of Unown in the same room. But you can catch multiple copies of the same Unown in its room.