What kind of add-on are you looking to make? Weapon/Vehicle/Decal/Prints/Music/Maps/Emmiters etc.
There are various tutorials around the forums relating to each of these and the various programmes you can use.
WeaponsA great tutorial by Azerath can be found
here. This is an incredibly useful tutorial and I recommend you read it if you want to make a weapon of any kind and have not done things like this before. This uses MS3D as the main programme for modelling, which is a superb programme for beginners and uses a standard 3pt orthographic view for modelling.
MusicThis tutorial by E_net4 is a basic method of changing normal music into Blockland playable music.
Emmiters and SFXMuffinmix made a very useful tutorial on special effects
here. It covers the ways you can change your simple rocket launcher trail into something with much more eye love. Incredible (however quite long) tutorial.
Decals/PrintsA vector decal tutorial by Skele is located
here.
Wedges tutorial on how to package decals is
here.
Maps and/or Mission EditorA tutorial on how to install the mission editor is located
here. For map making/editing this is a must.
Stratofortress made a nice quick tutorial on how to spawn objects into your maps using the mission editor
here. Note - You need mission editor installed correctly first.
Badspot made a post
here giving just a quick note to mappers to not forget their .ml files.
VerticalHorizons map packaging tutorial is located
here.
Tool tutorialsBlender - Heedicalking is starting a tutorial class
here. You may wish to watch that topic for updates
Blender animation and making add-ons generally - Darkstar has made a great tutorial
here. This is explained in great detail and incredibly useful.
MS3D - Stratofrtress made a tutorial on making models in MS3D and how to keep them "economic"
here.
MS3D Flatshading - A MUST for releasing models, you have to flatshade your model (or get flamed, its your choice). But if you don't know how to flatshade, Trader made a nice tutorial
here. It runs you through, step by step, how to flatshade a model in MS3D
MS3D Weapon - Masterlegodude did a reasonable tutorial a while back
here and it has some very useful tips (make sure you read kaje's post two posts down).
There are LOADS more, these are just the ones I could quickly find off hand. I hope at least one of them helps.
Edit - Rotondo's post
here. Covers most of the main programmes you may want to use and also links to a few tutorials aswell.