Same here, the enter key has no effect in the puzzle menu. Opening the desired puzzle is more complicated than it should be.
On the puzzle menu, there's the list of puzzles, the detailed description of one puzzle, and the search box and related checkboxes.
There are two ways to open a puzzle. You can click on the puzzle in the list, or you can click the "Play!" button.
The description controls which puzzle gets opened.
One puzzle in the list is highlighted, but this does not necessarily mean you'll be opening that puzzle.
If the description of the highlighted puzzle is already displayed, then clicking on the puzzle in the list or clicking "Play!" will open the puzzle. You really have to look at the description to make sure it goes with the puzzle, since the puzzle's number and name is not repeated.
If the description of the highlighted puzzle is not displayed, clicking on the puzzle once will display the description, and then another click or "Play!" loads the puzzle.
Searching for a puzzle illustrates the problem. Currently, if you search for "hexamer", the hexamer puzzle is highlighted, and is the only puzzle in the list. The description remains unchanged when this happens, probably referring to the revisiting puzzle. Clicking "Play!" at this point gets you the revisiting puzzle.
An empty search result is likewise confusing. Search for "heptamer", and you get an empty puzzle list. The description remains from the last puzzle highlighted. Clicking "Play!" opens the puzzle with that description.
The current design is confusing, and makes it easy to open the wrong puzzle. The solution is to make the description match the highlighted puzzle automatically, without another click. Pressing enter should open the highlighted puzzle. With an empty puzzle list, the description should be blank, enter should have no effect, and the "Play!" button should be grayed out.