We only have one adjusted FWD modulus value for HMA layers, but I can only see input options in the form of a grid of frequency and temperature.
How can I use backcalculated modulus results from FWD testing to represent my existing condition using Pavement ME Design?
The FWD modulus results are an input for the existing asphalt layers. One value can be input and a grid is not necessary, as shown in your screenshot above.
You should evaluate the backcalculated values for the asphalt layers and make sure the data you have is independent of temperature. The dynamic modulus of any asphalt layer is dependent on temperature and if the deflections are measured over a wide enough range of temperature, the backcalculated values can be grouped by time of day and temperature, as long as temperature is included in the FWD data file. These are engineering decisions and need to be made by the pavement engineer.