I've had to do this and it's no fun. Here's what I have (note that I'm using Mink with Behat).
Make a step called "I press the enter key" or something similar.
Then, in the context, do this:
1. Find the element that you need using $this->getSession()->getPage()->find("css", <css_selector>);
2. Then, call your web driver's keyPress function, like this: $this->getSession()->getDriver()->keyPress(<element>->getXPath(), 13);
The "13" in the function call is the ASCII code for the "Enter" key. The <element> is the element that you selected in #1.
I hope this helps.