The Moshipedia can be accessed at any time from the World Map
by selecting the appropriate button on the touch screen. In this menu, you can see every Moshling that has been collected and read their descriptions. Selecting an empty slot will display a general hint informing you what needs to be done in order to obtain the missing Moshling.
Hints displayed for each Moshling:
Search the levels to unlock.
Collect Rox to unlock.
Beat a mini-game to unlock.
Unlock via StreetPass.