Problem converting stl to solid to work onit

Hi everyone

I am a newbee and so am probably asking a very basic question. Going through the help files has not given me an answer - so I hope someone has the time to guide me.

I am trying to import an stl files to adapt it within Designspark mechanical. When I import any stl file I get a wire structure on which I cannot pull faces, etc. The HELP tells me that I can select 'solid' in the import options on the open box. However, this options box only gives me the options of "connected mesh" or "simple mesh". I have tried several stl files and always get the same issue.

I cannot seem to find a way to convert a mesh model to a solid within the program.

Any basic help would be great, even if it means using some 3rd party conversion software!

Thanks for reading this

David