Firstly I am going to point you towards
http://www.hard-light.net/wiki/index.php/Ships_and_General_Mod_Creation please read through it all.
Specifically to this issue.
With tbl files only one can exists of each type at once, additional tables of the same type are ignored. Next a tbl has to follow a set naming convention, in this case weapons.tbl is the only file name that is accepted by the engine, anything else is ignored.
The next point and a massive one is please, please, please don't change retail files until you know what you are doing and save any changes so that they don't overwrite the retail version, the vast majority of mods use retail files, especially tables some where along the line.
Now about tbm files, these, combined with mod folders are a newbie's best friend, they allow for small files to be created to change existing elements of the game or add new ones, like your weapon without making any permanent changes to anything else it covered in the link above.
For this case in the mod folder create a text file called something like Spurple-wep.tbl (the "-wep.tbm" is important as this tells freespace that the file is a weapons tbm), then with the file open start the file "#primary weapons" (without the speech marks) paste your entry next and stick #end at the bottom of the file. Save and run
