When using Google Chrome with Creately for Fogbugz the diagram editor can appear in a small window and therefore becomes impossible to use. After testing this seems to be an issue only with Google Chrome - Firefox and IE etc work fine. More precisely it seems to be something to do with the Pepper Flash player that comes bundled with Google Chrome. As a workaround if you disable the Pepper Flash player and use the Adobe Flash player the issue goes away.
This article describes how to disable the Pepper Flash player in Google Chrome: https://support.creately.com/entries/21725820-Creately-frozen-in-Google-Chrome