Due to the way scripts are captured in AgileLoad (at the HTTP level), the script must be able to handle session parameters and similar variables in the same way that the browser that it is emulating would.
To that end the script must be parameterised. This parameterisation process can be automated by teaching AgileLoad how to do this in its models. However before doing this you will need to understand the parameterisation process which is explained here.
This video shows step by step how to parameterise a value in a script in AgileLoad.