MiFi is a trademarked name for Novatel Mobile Hotspot devices. Jetpack is a name from Verzion used to sumarize thier Mobile Hotspot devices. All MiFi's are considered Jetpacks, but not all Jetpacks are MiFi's.
Naming aside the MiFi (4510L, 4620L, 5510L) is essentially the same device as the Jetpack 890L at its core. There are a few features and limitations to each device but in general they all connect your WiFi devices to the Verizon wireless network to access the internet.
You might need to be a little more specific if you want to know exactly what the differences are between them. For example, can you able to connect to the Jetpack at all with your iPod or is it just the scrabble site/app that does not work. If you cannot connect then your iPod most likely has a password mismatch or is connecting over a technology that is not supported or configured correctly. If you can connect the iPod to the Jetpack and other sites work but the srabble site/app does not work then there may be something else going on.
Please try to collect and post some more information on the problem for us so we can narrow it down.