The Conditional option for "Start when items are created" and "Start when items are modified" is not currently available on Nintex Workflow for Office 365.
However, it looks like this option is on the way. Check this link on the Nintex User Voice site: Conditional Start – Customer Feedback for Nintex. Be sure to add your vote -- I did!
Eric is correct, conditionals are not yet available, but I hope we can see them enter the product soon. Until then, you can use logic inside of your workflow to halt a workflows execution until a field equals a certain value for example. The downside to it is that the workflow must start and keep running until that condition is met.