I want to design a small electronic circuit using Altium designer v14.3.9, and I need the microcontroller Atmel's 89C5131 (the symbol, not for PCB), but couldn't find it in any of my libraries. Does any one know where can I get it?
I assume you are looking for a schematic symbol, and perhaps footprint. If so, you should just make the required symbols yourself - then you will know that it matches the component you are using (and, for the schematic symbol, that it has the pins arranged (and named) in a way that will suit your use.)
I don't trust other people's libraries (even Altium's - I once found an IC symbol that bore no relation to the maker's datasheet!).