CFW install on PS3 DECHJ00A

Hi all.
I have a PS3 TEST DEBUG DECHJ00A 4.81 OFW.
I want to install Evilnat but I can't. I tried ps3toolset without success. I installed DEX OFW 3.55 then tried to install CFW Evilnat DEX 4.90 but I get an error during installation.
Do you have any idea of the procedure to follow?
Thanks in advance.