If you can’t seem to be able to host a multiplayer game of Magicka you might need to set up port forwarding on your router.

Here is some information on doing just that directly from Steam.

Required Ports for Steam

Question

Which ports do I need to open on my router or firewall for Steam?

Answer

Your network must be configured to allow Steam access to the following ports (in order from highest to lowest priority for QoS users):

Steam Client

UDP 27000 to 27015 inclusive (Game client traffic)
UDP 27015 to 27030 inclusive (Typically Matchmaking and HLTV)
TCP 27014 to 27050 inclusive (Steam downloads)
UDP 4380

Dedicated or Listen Servers

TCP 27015 (SRCDS Rcon port)

Steamworks P2P Networking and Steam Voice Chat

UDP 3478 (Outbound)
UDP 4379 (Outbound)
UDP 4380 (Outbound)

Additional Ports for Call of Duty: Modern Warfare 2 Multiplayer

UDP 1500 (outbound)
UDP 3005 (outbound)
UDP 3101 (outbound)
UDP 28960

Note:

Many university networks and proxies block required ports for Steam operation – please consult your network administrator to ensure the required ports are open if you are using a university network or a proxy. Ports required for Steam can not be re-mapped to HTTP or reconfigured to a custom port range.

https://support.steampowered.com/kb_article.php?ref=8571-GLVN-8711

Having issues hosting a Magicka Game?