Andrew is quite correct for general Markov models however in this case the problem can be solved by specifying the form of the equation on the NL command substituting the non-negativity and sum to unit restrictions directly into the specification of the equation on the EQ line. This can be specified as follows:
nl 1 / ncoef=3
eq y = a + (b*b)*x + (c*c*)z + (1-b*b-c*c)*w
The variables are y x z w and the coefficients are a b c.
The application of restricted least squares to solve this problem was first described in Lee, Judge and Zellner (1970) and produces consistent but not efficient estimates. Another method is by iterative GLS (see MacCrae 1977) designed to improve efficiency - also possible within SHAZAM.
Note also the example for solving Non-Negative least squares which also makes use of the NL command in a similar way.
However, an alternative approach (also proposed by Lee et al) is to treat this as a Quadratic Programming problem and you may then solve this using the QP command in SHAZAM (version 11 or later).