Struggling with creating LIDS

I want to perform Flowexpress/simulation on my microfluidics manifiold design - I 've been struggling for a while with it. I have a model in assembly and just can't seem to get the "create LIDS" option. I don't use SWs often but this type of analysis is driving me crazy -the help pages don't provide any actual guidance and it seems they were written by a robot! Apologies but I'm just frustrated by trying to get this feature to work. Any help/advice would be greatly appreciated.

SolidworksModeling And Assemblies