How do you handle manganese nitrate hexahydrate?
Manufacturers of chemicals can make life interesting for first-time users. What a boon the CRC Handbook is! Without even listing $\ce{Mn(NO3)2.6H2O}$, the CRC Handbook can solve your problem. It gives the melting point of $\ce{Mn(NO3)2.4H2O}$ as $\pu{25.8^\circ C}$. Undoubtedly, the melting point of the hexahydrate is even lower, but maybe not by much. In hot water, the tetrahydrate is "infinitely" soluble.

Heat your jar of hexahydrate to 30C in warm water until all the crystals are melted and withdraw what you need with a pipette. You might plan ahead and fill several small bottles with the hexahydrate for later use. I doubt that the material needs to be stored cold.