These instructions are only tested with Salient v7.0.7.
When you try to enable Max Mega Menu with Salient you will run into many issues due to Residual Styling (for example: sub menus not displaying correctly, mobile menu displaying incorrectly, icons misaligned).
The easiest solution is to strip out/simplify the header. This will disable the Logo, Cart and Search functionality in your header, and replace everything with a single full width mega menu.
Download Salient Child Theme: salient-mmm
Here is how the menu looks immediately after enabling the Salient Child Theme and Max Mega Menu for the primary theme location:
To replicate the default menu styling, go to Mega Menu > Menu Themes and set the following value:
Menu Bar > Background: Transparent