There could be two reasons behind it,
Please note that Structural Synchronizer is the engine that makes ISM work.
1) In case there is a mismatch in Structural Synchronizer (64bit) and RAM Structural System (32 bit), download the 64 bit version of RAM Structural System by following the steps stated in the link below. Starting with version 15.05.00.00, only 64 bit versions are available.
Software Download Instructions
2) If you have installed Structural Synchronizer (64 bit) and RAM Structural System (64 bit) and the ISM says not installed, then there is a version mismatch between RAM Structural System and Structural Synchronizer.
Uninstall Structural Synchronizer from the machine. Download and install the latest version of RAM Structural System and Structural Synchronizer.
They can be downloaded from Software Downloads.
How can I find the ISM installer?