I also had to setup some Dell PowerConnect M6220 switches; these proved to be slightly more complicated than the HP's, but followed the Cisco style config.
First we need add a custom login and enable method
aaa authentication login "LineName" radius local aaa authentication enable "LineName" none
These are used for connections over SSH - "LineName" can be anything, I tend to use the company name to keep things standard.
On the second line, you could also use radius local - this would require a password to enter enable mode. Personally I control the login mode from the RADIUS server and find having to enter a password to get to enable again a hassle.
Next we need to setup the RADIUS server
radius-server host auth serverIpHere name "server1" usage login key "SecretKeyHere" exit
Now we just need to configure the ssh line to use our aaa methods
login authentication LineName enable authentication LineName
Once complete you should be able to login via ssh using your RADIUS details.
If you require access to the switch over http(s) you can also configure the HTTP server to authenticate against RADIUS
ip http authentication radius local ip https authentication radius local
We include local on every method to ensure we don't get locked out the switch.