I figured out why I couldn't install it, you have to install the New Bootloader FIRST! now I'm using this rom! The easiest way to do it is with the All-in-One tool for LG P990 you can find it in the XDA Forums.
Yeah, I will try unrooting, I know it's not your fault but I'm frustrated because I cant get the new MIUI in my old phone... Thanks for trying to help though.
I tried to install and as with Phil K, after factory reset, dalvik wipe and flashing rom, I get back to my old rom, the rom flashing process seems unusually fast, maybe there is something wrong with the file...