You can either set the values at the level of the project or of the model (in the MSL implementation of the relevant class).
1) at the project level
In the Block Details, enter the values in the Initial Values column - in fact, the only one that accepts inputs
2) at the model level
As I explained in another thread, the M object (in the standard model library) in defined in the wwtp.base.msl file (at around line 96).
If you are working with one of the standard categories, you may alter the default values indicated for that particular category (confined between #ifdef <CategoryName> and #endif).
If it's a custom category that you are working with, you'll need to add a whole block of code, similar to the ones you see for the default categories - and indicate the appropriate values.
