if you don't want to mess up your current dev environment, you can just run:
$ /Applications/Xcode6-Beta.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swift
Alternatively:
sudo xcode-select -switch /Applications/Xcode6-Beta.app/Contents/Developer
then you can do one of these two:
xcrun swift
lldb --repl
via :click