As Sven says, wrapping [IKeyPress, 13, ] around a command that returns a dialog box will prevent it appearing. But for loading a ztool this shouldn’t be necessary:
[IButton,"My ZScript",,
[FileNameSetNext, "C:\models\mesh.ztl"]
[IPress,Tool:Load Tool]
]
This code should work fine, providing there is a model called mesh.ztl. (Although apologies for the mistake with the Load Tool button path.) You can define the ztool as a variable if you wish:
[VarDef,model,""]
[VarSet,model, "C:\models\mesh.ztl"]
[IButton,"My ZScript",,
[FileNameSetNext, model]
[IPress,Tool:Load Tool]
]