I am trying to convert a FLV movie to QuickTime.
First of all, what do you mean by "Quicktime" here? Technically, virtually any file that will open/play in QT 7 can be turned into a "QuickTime" file/movie without recompressing the data by simply using the "save As..." option to copy the original data to an MOV file container. (I.e., an MOV file, whether or not the data encapsulated within it is "natively" QuickTime compatible is considered to be "QuickTime.") QT or third-party converters which can open/play FLV files can normally recompress/convert FLV files to other "native" QT compression formarts or any third-party compression format for which your system/player is configured.
I have QuickTime 10 and QuickTime 7 Pro. I have also installed Perian.
QT X and QT 7 with Perian v1.2.3 installed should normally support FLV files encoded as H.263 (a.k.a 'Sorenson Spark' or 'FLV1') video with MP3 audio under Snow Leopard, Lion, and Mountain Lion operating systems. So the next question would be what compression formats do your FLV files contain? (E.g., I don't believe PV6 or H.264/MPEG-4, Part 12 are supported by QT in the FLV file container but since I don't use them, I cannot confirm this.)
The video plays perfectly with flash player. But QuickTime ( 7 and 10 ) tells me that it cannot open the FLV file.
If your FLV files contain H.263/MP3 data, then the next question is, "What is the default player assigned to the "FLV" file extension class of movie objects and/or how are you attempting to open the FLV files?" I.e., if you are trying to double-click open the files, try changing the "Open with..." property or open the files from within the opened player app or try "drag 'n drop" opening the files or try the "open with..." context menu opotion.
Of course, if his current installation of Perian were working properly then he would not need iMedia Converster since QT 7 Pro (which the original poster states is also installed) could perform the conversions.