Ok, first, download Milkshape if you don't have it. (Search in google.com)

Now, open Milkshape and make a Box.
Click on tools/Quake III Arena/Generate Control File.
Name it "sqr" (sqr.qc)
Put it in the folder you want (but the folder must be inside the ".../GameData/Base" folder. For example:
.../GameData/Base/MyModels/sqr.qc

This is the file:

_______________________________________

// Quake III Arena MD3 control file, generated by MilkShape 3D
//
$model "models/players/model/model.md3"
// reference frame
//$frames -1 -1
// frame 1-30
$frames 1 30
$flags 0
$numskins 0

// you can have one or no parent tag

// tags

// meshes (surfaces)
$mesh "Box01"
$skin "models/players/model/skin.tga"
$flags 0

_________________________________________

We must modifie it.
First, look at the third line:

$model "models/players/model/model.md3"

This is the model's folder. It isn't the full path. It starts after ".../base" folder.
For example, if you have your qc file in
".../GameData/Base/MyModels/sqr.qc" you must change the line to:

$model "MyModels/sqr.md3"

(the folders after ".../base")


Ok, now look at the last lines

$mesh "Box01"
$skin "models/players/model/skin.tga"
$flags 0

Our model is only a box so there is only one mesh. This tells the md3 the texture to use with each mesh.
Get a texture out from assets0.pk3 (or use any texture you want) for example, I will use "bsic.jpg". Put your texture in the same folder as the qc file.
Now change the $skin line to:

$skin "MyModels/basic.jpg"

Notice that like in the third line, it isn't the full path.
Now, save your qc file.


In milkshape click on "Export/Quake III Arena Md3..." Export it in the same folder as the qc and the texture files.

Now you should have a completed md3 ready to use


Open jk2 Radiant, create a misc_model and search for your md3.
An error message will appear.
In the Entity editor, delete, in the model's value, from the start to ".../base/"
(this included) for example, if the value is: ":/Program Files/Lucas Arts/Jedi Outcast/Game Data/Base/MyModels/sqr.md3"
change it to just "MyModels/sqr.md3"

An extrange thing: In the path, the first bar must be "\" and the other bars must be "/"
For example, this will work:
"MyModels\objects/simple_objects/sqr.md3"
but this not:
"MyModels/objects/simple_objects/sqr.md3"

So, change "MyModels/sqr.md3" to "MyModels\sqr.md3"

Hit enter and luck

If it doesn't work:

1) IMPORTANT THING: You must generate, modifie and save your qc file BEFORE exporting your md3.

2) Here is my modified qc file:

________________________________________

// Quake III Arena MD3 control file, generated by MilkShape 3D
//
$model "MyModels/sqr2.md3"
// reference frame
//$frames -1 -1
// frame 1-30
$frames 1 30
$flags 0
$numskins 0

// you can have one or no parent tag

// tags

// meshes (surfaces)
$mesh "Box01"
$skin "MyModels/basic.jpg"
$flags 0

_________________________________________