I got myself into a state where App Store says that I have numbers "installed" but in fact I don't have it installed. I tried the recommended option/click and shift/click over the installed button and it didn't work. Can you give me a tip?
Reinstall Numbers
iworknumbers
Related Solutions
This screen shot is from Keynote for iPad, but Numbers for iPad is pretty much the same:
When you view the file in Preview mode, the title bar of the document should look pretty similar to the above. Over there on the right is the Open in… button.
There might be a 
button that you have to click first before you see the Open in… button.
Other ways to transfer files to your iPad:
iWork can still be bought online. Here are search results for iWork on Amazon.
Best Answer
Try resetting the Mac App Store.
Enable the debug menu and restart the Mac App Store.
Choose the Reset Application option.