Is this not necessary within Mplus? For instance in example 11.4 (see output file on mplus website) is the average intercept accounting for dropout 1.903 and the average slope -1.123? So I do not need to use the estimates of I on d1-d5 post estimation to calculate the average intercept?
The intercept of e.g. i is not the mean of i. If you want the mean i and s you have to take into account the effects of the d variables. You can do this using the Model Constraint command with parameter labels given in the Model command.