The Salvage computer must have Salvage Modules linked to it in order to be listed there. Hit C to select the Salvage Computer and hit V to highlight any placed modules. You can also C select the computer and then place down the modules. They will then be linked automaticly.
Beware of the green build arrow. Your Salvage computer must point foreward or the modules fire in another direction. Use Left CTRL Mousewheel to rotate a block.