UPDATE : For installing developer betas just insert this command into terminal and you're good to go
Code:
sudo /System/Library/PrivateFrameworks/Seeding.framework/Versions/A/Resources/seedutil enroll DeveloperSeed && /usr/libexec/PlistBuddy -c "clear dict" -c "add :SeedProgram string DeveloperSeed" "$3/Users/Shared/.SeedEnrollment.plist"
And then acces this page in safari : macappstores://itunes.apple.com/app/id1209167288
Credits to
v1597psh