Go to ExtJs website and make sure you download and included all the necessary files and IN ORDER,also using a newer version might break the code. try it with simpler examples and only reference the ext stuff and see if it loads
there is a firefox extension called "jsview", it allows you to see what scripts and css files are included on a page.
When navigating to the link you include at the top of your post and opening the ExtJs file, you'll notice that the example provided is using version 2.1 of the code.
The Ext.ux.TinyMCE v0.6 has a corresponding blog post here. You will notice that it specifically requires:
Be aware that when plugging in a newer versions of the TinyMCE or ExtJs libraries, there might (read will) be breaking changes that you will then have to resolve. You could try scrape the files directly from the example to ensure you have a working version.
If you choose to use ExtJs v3+, get the latest components:
1) ux.ManagedIframe has been maintained more recently and even upgraded for ExtJS 3.x here
2) Read through the ux.TinyMce thread and download the latest version. I think it's 0.7b. If you're still having problems, that's probably the best place to find answers.
Even when including ext-all(-debug).js, you still need to include the ext-base.js file before that, otherwise you will get the "Ext is not defined error". Not sure by your last comment if you are already past this or not, but I thought I would point it out. Please see this page for complete details on how your includes should be set up:
copy the lib folder in your eclipse workspace`s exjts project folder and just give the relative paths, is should than work