The easiest thing to do is use addpath() once to add the location of the .slx file to the search path.
Alternatively, add this code to the model PreLoadFcn callback
addpath(fileparts(get_param(bdroot,'FileName'));
And in the model CloseFcn callback
rmpath(fileparts(get_param(bdroot,'FileName'));
This approach will add the folder with .slx file to the path and keep it there until you close the model.
Slightly more complex approach that modifies the directory during Run or Update (which may be problematic, actually) and then reverts when the simulation stops.
In the model InitFcn callback use the following code:
current_dir = cd; % save for later
sim_dir = fileparts(get_param(bdroot,'FileName'));
cd(sim_dir);
Note: current_dir and sim_dir will be created in the base workspace. So use variable names that you know will not step on variables that already exist in the base workspace. Also current_dir will be used later (see below) and they both will be cleared when the simulation stops, so make sure you choose variable names that won't be overwritten by other simulaton outputs to the base workspace.
In the model StopFcn callback use the following code
cd(current_dir);
clear current_dir sim_dir
I tried this with a simple model using the Run button and it seemed to work. However, be careful if using the model in question as a Model Reference. Also, there may be other gotchas if using the sim() command.
