If you look everywhere and are careful, you will find all available weapon upgrade stations (power to the people) I couldnt find one even when I searched everywhere and finished the game. I found out this tidbit when I did a google search and learned that I needed to do one thing before doing two things. 

The Fort Frolic Level, when Cohens masterpiece is completed, save the game. Kill Cohen and photograph him for a the Irony achievement. Reload the previous save and do not kill Cohen, continue the game. Later in the game, you will arrive at an apartment level where two people are dancing, cap those suckers and Cohen will unlock his door to fight you. Up the stairs and in his apartment, on left side (entering his apartment) is a Weapons Upgrade Station (the one I couldnt find) and later in the game, when all weapons are upgraded, you will get the Weapons Upgrade achievement. Also you will get the Cohens room achievement when you go into his upstairs apartment.