I will try to step you through this...
1. Open up your mash profiles folder: 'profiles' >> 'Mash Profiles'
2. Click icon for adding a new mash profile: 'Add Mash'
3. Set the grain temperature for a typical value you hold your grain at before adding to the strike water
4. Towards the bottom of the pop-up screen under the heading of 'Brew in a Bag (BIAB) and Full Boil Mash', click on the box next to 'BIAB Mash with Full Boil'
5. Now, just below the step box, click on 'Add mash step'
6. Make sure the type of step is set to 'infusion'
7. Set the 'step temperature' for the mash temperature you wish to target
8. Change the 'step time' to the length of your mash
9. Enter a reasonable time in the 'Rise time' box. I have 10 minutes for rise time to account for the time I spend mixing the grains into the water and ensuring there are no grain balls
10. Click on 'OK' to save the step.
11. Give the profile a unique name so you can find it. Try Gavin's recommendation of starting with a number to move it to the top of the list.
Now you have a profile that you can use to brew with. The numbers will change based upon the size of your recipe.