I've made MSP externals for Max5 and Max6 with the SDK of Max 6. With Max 6, everything is working perfectly but I've a problem with Max 5. Max 5 can't load the external because it doesn't find gensym_tr in the MaxAPI library. It's seems that I'm not the only one :
Instead of my objects, simplemsp~ is working fine on both versions, I don't know why. If someone got a solution, I will be pleased to know it, I don't want to create an object for each Max.