As of a couple of weeks ago, the Export dialog box is not appearing when I click Export. Nothing comes up. This is on a Mac with Yosemite and with the latest LR CC. The various Export presets I have already defined work but not the main Export dialog box where you can set your export parameters and create a new Export preset. I'm wondering if there is a conflict with other export presets I have installed from third parties, such as LRTimelapse.
Well yes that could cause this, can you try to remove the 3rd party preset and them try the same.
There is a known nasty bug with the latest LR CC and the export panel that especially rears its ugly head on Mac OS X. Try removing that preset, but I highly doubt that is the issue. You probably need to revert your Lightroom to CC 2015.1.1. See here for instructions: http://www.lightroomqueen.com/how-do-i-roll-back-to-lightroom-2015-1-1-or-lightroom-6-1-1/