FYI if you have a weapon equipped it will show as "no options available"
Here's a pretty lengthy, but very informative video on how weapon modding and the Salvage Matrix works!
http://www.youtube.com/watch?v=ZO74ITJ2af8
Keep in mind, also, that the Iron Demon Ranch has a ton of low level mods available for any weapon if you just want to mod something. Find out which mod slot your weapon has, find the corresponding weapon mod for that weapon type in the merchant, and you're in business.
Only buy the ones you need for your weapon types. There are mods for all weapon types, but they will only fit a single weapon type. Don't throw money away buying mods for detonators if you only use assault rifles and shotguns.