I rechecked my blog and looked at Network Admin page & found the theme I had added to Hostgator in a list of installed themes. I also noticed at that page at the top was a button to 'Add New' (theme). From this page I had to activate the theme. After doing this the new theme showed up in my list of available themes on my blog dashboard>Appearance>Themes and I was able to install it.
So this work around works & has solved the immediate problem (though why the 'Install' tab has been removed from Theme page in this new version is beyond me & seems unhelpful-but at least there is a way to get around this-although this seems more complex & time consuming & less simple).
I hope my experience may help anyone else who finds their 'Install' tab missing.