What kind of parameter is it?  Are you talking about an " output parameter"?

To be more specific, for say BSIM3, "vth0" is a parameter, you can set
.model nch nmos level=49 vth0=0.9

There are also what Spectre calls output parameters, eg, gm, gds, etc.; you can't set these (the model computes them), but you should be able to access them for plotting.

And, when you say "access" do you mean plotting, or are you trying to access the value of beta to set something else?  The latter is not possible if your beta is an output parameter; if it's an input parameter, you have to declare a subckt parameter and use it in both places.

It could even just be that you're referring to the model name rather than the instance name, eg

.subckt mysubckt
m1 (d g s b) mosfet ...
.model mosfet nmos level=49 beta=5
.ends

x1 (d g s b) mysubckt

you should be referring to x1.m1.beta; mosfet doesn't appear in the name.