Blichmann kettles are included in the equipment profile add ons. Choose the one you need for your profile... 20 gallon HLT if memory serves... and use that as the foundation to create a custom profile.
You will need to make a custom profile no matter what if you want reliably close estimates of volumes, gravity etc. on brew day. If you don't you will have nothing but frustrating brew days ahead wondering why your OG is too low or too high or volumes going into the fermenter will be off from what BS predicted. That's because the profiles included in Beersmith were created and submitted by users. Those profile fit that brewer but your processes are likely different enough that they won't fit the way you use your system. Even Brad Smith recommends you use the included profiles as starting points to customize your own.
Here are two tutorials to walk you through setting up your own profile. Both are reliable but the first one I believe is the most comprehensive.
https://youtu.be/HwEbjOt8OR8
https://youtu.be/QmW7pwQP5mQ