Okay, from the beginning. First of all, you are asking the wrong question

The problem isn't "How do I add ships to FRED" the real question you are asking is "How do I add custom ships to a mod", or "How do I create my own mod". Because once you have ships in a mod, and you load up FRED with that mod active, all those ships will be there for both FRED and FS2 to use.
Now, first, you need to understand how to build a mod without ruining your FS2 install in the process. The first rule you should keep in mind is that the retail FS2 files are strictly "look, don't touch". You can, and indeed are encouraged, to unpack the vps, and take a look at the files contained therein, but don't, under any circumstances, alter them.
Second, in order to create a new mod, first make a new folder in your FS2 directory. Name it "Mymod" or whatever. In that folder, create an empty text file, and name it "mod.ini". This little file will tell the Launcher important things about your mod, like what other mods it depends on. If you wanted to always load the mediavps, your mod.ini would look like this:
[multimod]
primarylist = ;
secondarylist = mediavps_3612;
Now, in order for your mod to actually have data in it, you need to create a subfolder named "data" in the "Mymod"-directory. Please refer to the
wiki page on FS2 Data Structures for further in-depth information on what goes there.
If you want to add ships, the important subfolders are data/models, data/maps and data/tables. These need to be created.
About ships: Every ship in FSO is made up of three parts. First is the actual 3D model file, also known as the ships' pof (Because the file extension is .pof, you see). This needs to be in data/models. In the pof, there's a list of textures, all of these (including the -glow, -shine and -normal variants) need to be in data/maps. Finally, there's the ships' table entry. This must be defined either in ships.tbl, or in a -shp.tbm. The latter is preferred for mods that are built on FS2, or other mods. For a further explanation of the difference between tbms and tbls,
refer to this wiki page.
This is basically all you need to know about adding ships. There will be no example here, as everything can be found by reading the documentation on the wiki, and looking at other mods' tbms as a guide. Good Luck.
EDIT: "no_fred" is a very special ship flag that is only used for one single ship, the Volition Bravos. It should be obvious why.