In order to use a trial version, you need to have a version of MATLAB that is the same version, in order to do the installation. A trial version of MATLAB is acceptable for this purpose.
Once you have the current version of MATLAB and the current ICT installed, you can use the trial license file to run the older version. You might be able to go to your My Account page on MATLAB Central and use it to download the previous version; I am not certain the Trial license configuration supports that. I know that if you have a regular license for a toolbox then you can use the My Account page to download any previous version back to about R12.
However, the Trial licenses are granted under the condition that the possessor is evaluating the software with an aim to buy a current version. If you were to buy a current version of ICT you would need a current MATLAB to run it on. Mathworks has usually permitted people to purchase toolkits for versions no more than about 2 or 3 releases back. As you are going to need R2015b MATLAB to run your R2015b ICT you should be testing your software using R2015b MATLAB not R2011a. Even if you had an R2015b ICT installation you would not be able to use it with R2011a MATLAB, so you should be investigating what it would take to upgrade your entire software to R2015b including the ICT.
