See here for how to get the LUI binaries.
Debugging Features in LUI mode
The LUI mode can be activated with the -l option, assuming yzlui exists on PATH (also see AceFaq). While in LUI mode, lots of additional debugging features are available as "colon" commands:
get the parse chart of the last processed item
display the AVM of a grammar rule (e.g. :r sb-hd_mc_c)
display the AVM of a grammar type (e.g. :t synsem)
display the AVM of a lexical entry (e.g. :l sleep_v1)
:g one two
compute the GLB of types one and two (e.g. :g + -); results will be shown as text, not in a GUI
show the local supertypes and subtypes of type (e.g. :h head); also shown as text
in generation mode, show which trigger rules fired and why; when rule is provided (e.g. :trigger does1_pos_rule3) , it will show more information about the particular rule (including why an unused rule didn't fire); results are shown as text
Getting the yzlui binary on your PATH
On Linux, you can get the yzlui binary from within the LOGON tree. You'll need to copy it to somewhere like /usr/local/bin/ or else modify your PATH environment variable to point to the directory containing yzlui. You can also download a (relatively recent but not necessarily maximally up to date) yzlui binary from here (again, it will need to be renamed 'yzlui' and put somewhere visible to PATH).
On MacOSX, the situation is slightly more complex and much less tested. Download yzlui for OSX here, and follow the INSTALL instructions. Good luck!
A few further tips for those not too familiar with Unix systems and terminal commands:
Suppose you downloaded the archive to your Desktop. You can typically expand it by double-clicking on it, but an alternative way is to open the Terminal application (use Search if you can't locate it immediately: the looking glass icon in the upper right corner) and type:
tar xzf yzlui-for-osx.tgz
Then to follow the instructions:
sudo cp yzlui /usr/local/bin
(it will prompt you to enter your administrator's passowrd)
sudo cp *.dylib /usr/local/lib
sudo cp -r yzlui.app /Applications
If the last stop doesn't work, this maybe because Mac hides the extension of the file. In this case, note which file appears in the Finder as type "Application" and move it to the Applications folder manually.