Trying using the macro to run your asset mix. The goal seek appears to be working, but the target doesn't seem to update unless the macro is being called. Keep in mind those numbers are specific to an asset mix AND a scenario (the latter is hidden in the magical innards of the model). That number shouldn't be used when making recommendations since it represents 1 possible economic path of what what I would guess are 1000 different paths. Look instead to the metrics on the Results tab since these take into account all the economic scenarios which were run for a particular asset mix.
See also - Monte Carlo Simulations