Regarding covariates, I would recommend to use the graphical model builder since it is much simpler, but if you want to do the textual coding, you start by defining the covariates, e.g.
fcovariate(Wt)
fcovariate(Sex())
​where Wt is a continuous and Sex a categorical covariate.
Now you need to specify covariate effects. If you want to check an effect of Wt on Volume you need to update the structural parameter:
stparm(V = tvV * wt^dVdwt * exp(nV))
the new covariate effect dVdwt needs to be defined as well:
fixef(dVdwt(enable=c(0)) = c(, 0, ))
If you want to practice I would recommend you do a few steps in graphical building mode and check the corresponding PML. This will help you to quickly learn the syntax.
Bernd