What version of 2d toolkit do you have? I'm running 2.5.0 and 2.5.4 and it's working fine in both. Perhaps your running a version of tk2d that was released before Unity 5.2? (Note the asset store version is quite slow to update, you might want to grab the latest version here from the forums if that's the case).
What happens if you try opening the test demo at "TK2DROOT/tk2dUI_demo/2 - UI Layout Demo" and try using the sample text box. Does that one work?
One thing to check is the actual input textmesh game object in the hierarchy (in the demo its under the input control "TextInputWithButton" as "InputText"). It could be the text is actually updating but there's an issue with your font or UI camera that's preventing it from being rendered.