The border radius options are available from BM Content builder v 3.15
It works like this:
The border radius options are available on the Advanced settings. The Enable Corner Radius should be set as ON, then the 2 groups of input-boxes will display, the 1st group is for desktop, and the 2nd group is for mobile layout.
You need to enter a number for each box, each number is for one corner(Top Left, Top Right, Low Right, Low Left) of an element. The number is a length for the corner, You can enter a enough big number like 999.