Cimatron is always making a background assumption about which solid object in the file is the active object - the one that add or remove solid
would apply to.
This mechanism generally works very well, but on rare occasions it will need some user input.  If Add and Remove are not available choices, the system does not understand the active object, and it only offering to make a NEW solid.
Correct this issue by using the Activate Object function from the Solids menu, and manually select the solid to be worked on.

*NOTE - Most functions from the solids menu will let you select the active object right on the fly.  Use "Pick Reference Object" in the optional steps area of the feature guide.