Codec & Format, with 2 links inside to read for general informaton
-read both links in reply #1 http://forums.adobe.com/thread/1270588
Report back with the codec details of your file, use the programs below... A screen shot works well to SHOW people what you are doing
This is a old matter which seems to find the same answer which is not looking for and installing codecs.
The answer is the Premiere Elements does not support the import of .flv files (never has) although you can export a .flv and f4v file from Premiere Elements (using in your case of Premiere Elements 10, Share/Computer/Adobe Flash Video).
This is an old topic. One of the more recent discussions of it can be found in this 2012 thread here
A Moyea plug-in used to work as a plug-in for Premiere Elements so that it would be forced to accept .flv files, But I have not found that possible in later versions. I will double check that anyway.
But the answer is....Convert the .flv to another format and then import the new format into Premiere Elements 10 project.
Add on...I think this is the Moyea Plug-in mentioned. I suspected that it worked with Premiere Elements up to version 8.0/8.0.1.
I remember working with it successfully in the days of Premiere Elements 4.