Installing Lion on a second machine

All not OK, all I get is a box indicating the 'candy' bar (blue) and the words 'checking "(null)".
This goes on and on. I returned to the App store and I assumed that I could simply download again from the store and ignore the version on the memory stick - not so the store tells me that it is installed and I cannot find a way to download again. Can anyone help me please?
John Marriott