There is a bug in latest versions of DO 9.1.1.6 through 9.1.1.8 which prevents (on my computer-XP Pro-32-bit) the dragging and dropping of files displayed in the file panel into zip files displayed in the folder tree. Windows reports that path or file is not found (3). I have reported this to Dr. Perry and he is working on the problem. He suggested I uninstall any other programs which might change the Windows system default handler (Zip-it-free, ExplorerPlus, 7 Zip, WinRar, jZip, etc.) and also verify in DO Preferences section that making DO the default zip handler is checked. After performing all these steps with DO 9.1.1.8 the problem still remained. I uninstalled ver. 9.1.1.8 and reinstalled version 9.1.1.5 and the problem disappeared. If anybody else has this problem, please let me know. I have not tested versions 9.1.1.6 or 9.1.1.7. Nothing beats DO for file managment that I have found, but I wish it could play .flv files and add support for 7z, CAB and other archiving formats in addition to RAR!
Opus will play FLV files if you follow this FAQ. You probably just need to install an FLV splitter (and FFDShow if you don't have it already).
I can reproduce the zip problem, on my Vista x64 machine at least. I'll talk to GPSoftware to see if I can help find the cause.
BTW, it's version 9.1.1.8 (etc.) not 9.1.1.18. I've edited the version numbers in your post to avoid possible confusion if those higher version numbers are used in the future. 
Thanks, Leo for the .flv info. Sorry about the version numbers!
Hi Leo, Greg:
Just tested 9.1.1.6 on another computer and that version seems to work fine. Must have started in either version 9.1.1.7 or 9.1.1.8.