How to get the gmail new compose experience if 'Try out new compose experience' is not available ?
It's already the default for users.
Gmail's new compose now default
You are busy people, so it's no surprise that an overwhelming number of you opted to try out Gmail's faster, simpler compose experience after it launched last October. In addition to telling us what you love about the new compose experience (like how much easier it is to multitask!) you’ve also been sending us helpful suggestions for what features you'd like to see added. As a result of your input, we're now ready to introduce the new compose experience as the default for everyone. We're looking forward to hearing what you think!
If you're already using the the new compose experience, you've probably noticed it's gotten a lot of upgrades over the last few months, from a new way to send files with Google Drive to much-requested features like pop-out replies. We’ve also added support for originally missing features like starring and labeling when composing and the Canned Responses lab.
The new compose will be rolling out to everyone over the next few days. You can learn more in our help center, or follow us on Google+, where we'll be posting more tips for using the new compose.