The 'Cannot open Multi.../default.mlt_omn' error message signifies that Omnisphere or Trilian could not find a correct STEAM folder, or that the STEAM folder is not the correct size.

Refer to the library sizes in this post to ensure your Spectrasonics instruments are fully installed.

Some users store their STEAM folder on an external or non-system drive - if you are doing so, and your STEAM folder is the correct size, but the above issue persists, the alias or shortcut to the STEAM folder in your Spectrasonics folder may be broken or incorrectly set up. 

To fix this: 

• Follow the instructions in this post to locate your Spectrasonics folder
• Within the Spectrasonics folder, delete the STEAM alias (Mac) or STEAM shortcut (Windows) you find there
• Load Omnisphere or Trilian in your DAW/host or standalone application
• When you are prompted to locate your STEAM folder, select the correct STEAM folder on your external or secondary drive