I've just given it a try. For me the installer is able to locate the packages on the stick. So it should work, I guess, though I don't think this installation method is officially supported by Univention (judging from other posts here in the forum and from a support database article that concerns this topic being "under review"). That being said…
Your screenshot proves that you've written the image to the pen drive correctly. Otherwise the installer wouldn't even have been started. If the installer cannot find your pen drive then a lot of different issues can be the cause. You have two options: debug this with a dubious outlook for success or simply burn the image to a DVD and use that for installation.
If you want to write the image again then here's how I'd do it: on Linux I simply use dd, e.g. "dd if=UCS_4.1-0-amd64.iso of=/dev/sdc bs=1M" (pay attention to the device name! Make doubly sure you don't write to a fixed disk!); on Windows I'd probably use something like ISO to USB.
If you want to debug it a bit you should run the installer up to the moment it cannot find the image. Then switch to a second console by pressing Ctrl+Alt+2. Activate said console. Then have a look at the syslog with "more /var/log/syslog". Look for anything related to USB: which modules are loaded, which hubs are detected, are there any error messages. You should also pay attention to anything related to disks as your pen drive should be recognized as sdb or sdc or something like that with two partitions on it (sdb1, sdb2 etc.). For me it was found as sdc as I have to fixed disks in this machine.