Huh, odd. The forward history macro definitely works for me on High Sierra, so I guess the dictionary app on El Capitan has a different UI element arrangement? Strange that it would work for back though 
Unfortunately I don't have an El Capitan machine here to test on, so I'm afraid @JMichaelTX or someone else who's using El Cap and knows their way around UI scripting will have to troubleshoot. Sorry I couldn't be of more help!
Ironically enough, currently my process for this is entirely manual 
I've downloaded and tried @JMichaelTX's Upload Macro to New or Existing Topic macro, which works great (and I believe is based on that macro of @ccstone's that you use) but since I usually upload macros to existing topics, I've just found it faster to manually export the macro through the File menu and copy as image from the Edit menu, followed by running the image through @Tom's great Downscale Images for KM Forum macro (which I activate through an Alfred workflow for executing KM macros similar to Trigger Macro by Name, something I'm sure he wouldn't be happy about 
).
One of my automation to-dos is actually to find a way to automate this process, which is probably pointlessly reinventing the macro uploading wheel, but I'd like to get something that better suits my own needs (plus, I figure it will be a good exercise, since there's so much I still have to learn compared to the true gurus mentioned above). If you're interested, I'd be glad to share whatever I come up with once I finally get around to coming up with it 