How Do I Catch All The Unown For Pokemon Platinum?

The Unown are located in the Solaceon Ruins just east of (you guessed it) Solaceon town.  Certain Unown are only found in the larger connecting chambers but most are in the small rooms and there will be multiple types in those rooms.  Just wander around and you'll eventually find the entire alphabet.

Unown have a super high catch-rate, so any pokeball will work.  When you have the entire alphabet, the Ruin Maniac will finish digging Ruin Maniac Cave which is accessible from route 214.  There, you can find Unown question mark (?) and exclamation point (!).

In the Solaceon Ruins, the unowns are the only pokemon. The main room doesn't have any unown, but each main room has a specific unown (F,R,I,E,N,D) the other letters are found at random in any side room. ! And ? Are found by the other entrance, not in the main maze of rooms.