Anyone knows how to export favorites or where are them stored?I need to install reason on another computer and obviously I don't want to choose all the presets again

The favorites folders and their contents (patch aliases/shortcuts) are stored in the reason preferences file along with other program settings. You can copy and backup this file by saving the Reason Preferences.prf file located in your C:\Documents and Settings\your_username\Application Data\Propellerhead Software\Reason folder.

But remember this - the favorites folder only stores aliases to patches. So after restoring your preferences file after reinstalling Reason, all of the aliases to patches saved in your favorites folder won't work unless the patches they link to are in the same places they were before. So if you have favorites patches stored in a number of different places on your hard drive(s), you need to copy all of those patch files back to their previously saved/aliased locations.