Additives to allow wax to melt in the microwave?
Microwave heating works best on polar molecules, such as water and alcohols, as it then causes the molecules to rotate or "jiggle" violently. Waxes are nonpolar, so heat poorly, but if your application can use fatty acids in the mixture, such as palmitic or stearic acid, you might try mixing some into the microcrystalline wax. The dielectric constant is not significant in your application.

However, the longer the fatty acid molecule, the less effective it is in absorbing microwave radiation and converting it to heat, and on the other hand, the shorter molecules have strong odors and do not mix well with wax... do not use butyric acid for that reason. The suggestion of using an absorbent container, by @Li Zhi, seems to be more effective.