A quick little guide to importing the save files you'll find under resources, with help from PojeBunny:

• Open DiRT 3 and create as many new profiles as saves you want to import
• Close the game
• Number your save files as you want them ordered, there are 17 files for each profile which have to be numbered the same, edit each individual file's number to follow on from your already existing files
• Copy the now re-numbered files into the save file directory (wherever your steam install is \ userdata \ your profile ID \ 321040 \ remote)
• Open the game
• Your save files should be in the opening screen, assuming you've done everything right
• Edit the settings to your liking, as they will have the settings of whoever posted the files initially
• Enjoy