I've just installed CbB.  In my Mix bus I'd route to external hardware and back using External Insert.  I've done this since Sonar 8.5 and SPLAT.  In CbB the option is there but nothing happens when you select it. 
Is this function now gone?

Funny you should mention this.
 
I was trying the same thing also today.  I use my Lexicon MX200 as a External Insert to provide the vocalist reverb in their headphones. The External Insert feature has worked for me since Sonar 8.5 as well, but this is the first time I used it with Cakewalk by Bandlab. I never had a problem inserting the External Insert and getting it working using my saved preset. However, nothing has worked at all today when inserting my Lexicon MX200 as the External Insert.
 
OK Bandlab, what have you done to the External Insert in Cakewalk.

No changes to the external insert. Can you be more specific as to what you did and what isn't working?

This is interesting, a feature I have never thought to use.. Jesse I also have the MX 200 which I use in junction with a small Mackie mixer for my cue mixes. But I've always wanted to try patching it into Cakewalk. 

It seems to still work here just fine with the latest CbB.

Cbb's External Insert is fine here.
 
Check that inputs and outputs used by External Inserts(s) are not assigned to any other track, otherwise they will not be shown in the External Insert's I/O selection dropdown menu.
 
One more note: External Insert is found at the top level of the context menu, not in Audio Effects submenu (that's where I look it every now and then...).

HHHmmmm, strange.
I went into the studio this afternoon and turned on everything. I then started CbB and tried the External Insert again and this time it worked.  I selected my preset from the External Insert as I have done so many times before, but instead of not working like it did yesterday, this time it worked.  
 
Gremlins have entered the studio again this Christmas season.

 
OK, all is fine and the External Insert is working.
 
MicV,

What were you connecting as an External Insert into CbB?
Remember to read and follow the instructions for the CakeWalk Documentation

Cactus Music
 
This is interesting, a feature I have never thought to use.. Jesse I also have the MX 200 which I use in junction with a small Mackie mixer for my cue mixes. But I've always wanted to try patching it into Cakewalk. 

Catus,
 
A hardware reverb unit is great to use as an External Insert with CbB for Cue Mixes.
 
Follow these instructions from the CakeWalk Documentation to get started and Just remember to keep the Gremlins out. 

Noel Borthwick [Cakewalk]
No changes to the external insert. Can you be more specific as to what you did and what isn't working?

 
It's encouraging to see that it's still working for folks so it must be something on my end.
This is what I do, like always:
- On a track or bus right-click the FX bin and select External Insert from the pop-up menu.
The External Insert never get inserted in the FX bin.  Also I can't insert the Insert FX Chain either.  Insert an FX Chain From Template works however.
If I open a project that already has an External Insert there, then it would work fine, I can route re-route to available ports no problem.
So the issue is it doesn't get inserted into the FX bin.

That's is very odd that the Eternal Insert is not being inserted into your FX bin after selecting it.
 
Maybe bring up an older project that has the External Insert already on it and remove all tracks and buses except the ones with the External Insert and then "Save As" the Template as an External Insert Template.  
 
You could build tracks and buses around the External Insert to accommodate your new Template.
 
It's strange that you can use older projects with the External Insert already applied, however, you can't insert a new External Insert into your new projects.