How to check breaker compatibility?
The breaker you picked should work fine.  To determine it for sure, read the fine print on the label on the interior of the door panel.  It should list a series of 10-20 types of breakers that are compatible with your panel.  They are often 2 to 4 letter codes.  The actual breaker models will be that code, and then a number.

Duplex, Triplex, and Quadruplex breakers often have an incompatibility tab on them to prevent you from using them in some slots of your panel.  This is called 'circuit limiting'.  Your panel has 30 lots, and is "rated" for a maximum of 40 circuits.  Thusly, only the bottom ten slots (five rows ) "permit" you to install a duplex, triplex, or quadruplex breaker.  This practice is called CTL or circuit limiting.

You can either play along and use two of the five bottom rows.  Or you can search for a "non-conforming" breaker.  From Murray's product list, it's not clear that they make one.  The tab could also be physically ripped out or cut.  But of course that would violate code, and if you do it wrong, you could die.  And that's no fun.