This was a bit tricky for me to figure out, but I think I got it:
Show History 1.1.kmmacros (4.4 KB)

Here's the UI script that I managed to get working in fullscreen:
tell application "Dictionary" to activate
tell application "System Events"
tell application process "Dictionary"
tell window "Dictionary"
tell UI element 3
tell toolbar 1
tell group 1
tell UI element 1
tell button 1 to perform action "AXShowMenu"
end tell
end tell
end tell
end tell
end tell
end tell
end tell
And here's one for forward history too while I'm at it (as you can guess, it was a simple substitution of 2 instead of 1 for the key button element):
Forward History.kmmacros (4.4 KB)

Let me know if either of these doesn't work for you on your system. Hopefully they do!