Many USB WiFi dongles are notorious for not having proper Mac drivers. I surveyed them in a huge electronics superstore in Portland Oregon. They had six brands of USB dongles. None mentioned Mac support of the boxes. I jotted down the names and model numbers for later research. In web searching for drivers for there on the internet, I found four said, "no Mac support" and two that refering teh usr to third-pary sites for the drivers. That resulted only in circular links and no drivers.
If the dongle's website does not show drivers, you need to know the chipset they used. It is sometimes possible to find a generic Mac driver for units with a particular chipset.
The other option is to return the ones you have as non-working and order one of these from the US:
It will do wireless "n" on a USB 2.0 port, and the extension cradle offer flexibility in finding the best reception. The firm I linked ships to the UK. And it comes with Mac drivers.