1. (As Trivial Psychic said) You have to go into the model's .pof. There you have the option to add the data for where the firing points for the model need to be; a bank needs at least one valid firing point. In PCS, you add the bank first and then have the option to add the firing points.
If you are doing this to an existing ship, I highly advise you to save the result in a seperate file because banks that are on the model but not in the tables tend to produce an error message on start-up (and that muddles every debug).
2. Then you need to add the bank to the tables. Which generally just means finding the lines "$Default PBanks" or "$Default SBanks" respecitvely in the ships tbl/.tbm-entry and adding another data point in the ( [...] ).
Like so: [before - - - > after]
$Default PBanks: ( "Mekhu HL-7" "Mekhu HL-7" ) - - > $Default PBanks: ( "Mekhu HL-7" "Mekhu HL-7" "Mekhu HL-7" )
2a. If you also want the weapon banks to be restricted to certain weapons (FS_Open 3.6+ or newer), you also need to find the lines with "$Allowed SBanks:" or "$Allowed PBanks:" and add additional data.
Like so: [before - - - > after]
$Allowed SBanks: ( "Rockeye" ) - - -> $Allowed SBanks: ( "Rockeye" ) ( "Tempest" ) ( "Tempest" )
If you are using a seperate model you should be using a seperate .tbl/.tbm entry.
If you remove the "in tech database" from the $flags:-line it won't show in the tech room; anything you add to the name of the entry after a # should also be invisible during gameplay.
Edit: I really shouldn't type during a lecture