Natural frequency and mode calculation:mass or weight

I am trying to calculate natural frequency of steel structure using Eigenvalue method.

My unit system is "kg mm".

Now STAAD guidelines says

"The input you specify is weights, not masses. Internally, STAAD will convert weights to masses by dividing the input by "g", the acceleration due to gravity."

While specifying mass matrix, if my equipment weight is 100 kg what should i specify in input file?

Your statement mean to say that 981N (here Newton since weight but my unit system is in kg mm)

Or should i change unit system?

I am confused , please help.