Well, actually there are several ways to do that... first under tools/options click on the brewing tab. Then choose your brewing equipment, I have fix boil based on equipment checked because I have the equipment dialed in to my kettle. These are your defaults for your new recipes... so I set my default batch size at 5.25 and my brewhouse efficiency is 73%. So now when I start a recipe BeerSmith calculates my OG and hops utilization based upon that 73%. So start a new recipe. You need to have the 'Type' of recipe set to All Grain and then in the recipe you will see the Brewhouse Efficiency button and next to it you should see the default is 73%.
So leave the Eff at 73% and add a grain to this new recipe... add 10# of Pale Malt UK. If you look down at the Beer Profile section you will see the Original Gravity is 1.049. This is based on what 2Row UK Pale Malt can give and your brewhouse efficiency. Now instead of 73% put in 80%. Notice the OG went from 1.049 to 1.055, you had 7% better eff. so now it reflects in your OG. Now, I use this method only to tease myself to see if I had better eff. how much less grain I would need to get my 1.049. Because now I can dial back my 2 row UK to 9#, I saved 1# of grain to get my 1.049 with my new eff. of 80%.
The way I use it, and I think most people use it, during the brewing process is I set it up to where I know I can do, in my case 73%. I base my recipe around that 73%. Then after I'm done brewing I take my OG at the volume I told Bsmith I was going for, in my case 5.25 gallons. Then I click on that brewhouse eff button next to the 73%. There I can input my OG and see my actual eff. which I take note of and use next time I brew the same batch. It is usually w/in a point or so of my OG. You can also figure your Eff. into the boiler and into the fermenter. So I use this window to refine my Eff number that I use. Cause the idea is to be able to reproduce your recipes. Overshooting can be exciting but if you design a recipe and want to replicate it you have to ... well replicate it
Ok, third use of Brewhouse Eff. in Beersmith is if you see a recipe on here that you like AG brewers generally post their efficiency with the recipe. So enter the recipe exactly as you see it on here, even the exact boil amount, etc etc. even if your boil and eff. are different... change the eff on your recipe sheet when you enter their recipe to match their eff, change the volume to match theirs. If it is in metric go into Tools/Options and click on the units tab before you enter the recipe and change it to metric. Ok, so after you enter the enter recipe as written here you then click on the scale botton above where it says recipe view. Here you change the brewhoue eff. to what yours is and change the batch size to what yours is. Beersmith will now scale the recipe automatically to your eff and boil size. Now if you changed the units to metric to enter the recipe you can go back into the Tools/Options units tab and change them back to English (US) units and BAM you've just entered a recipe in metric with someone else's brewhouse eff and scaled it to your own equipment. Notice all the grain amounts and hops change and the OG and IBU (this is never exact when you scale but close) will all be changed to suit the new scaled recipe...
Hope this helped!