It looks like it was part of JuliaPro, which has been retired as a project. This Github README has instructions for installation and usage of JuliaInXL. But there hasn’t been much maintanence of the code on the Julia package side or the Excel plugin side, so it’s quite possible that it doesn’t work anymore.
There are several other threads here asking about JuliaInXL’s current fate: 1 2 3 4 but none of them have been answered, so it seems highly likely this is an abandoned project which shouldn’t be relied upon.