@Joe91 is correct, in Manager this is done via Batch create. To understand the detailed formatting required enter some data manually the use Batch update to see what batch create would have been required (which is the same as Batch update except has no Key field.
Most tabs in Manager corresponded to a database table. Order the data import to ensure you start with non dependant data, then importing data which references already entered data.
For further in formation
A forum search is a good place to start, such as Search results for ‘Quick books import’ - Manager Forum
The advantage of doing so is summarised here Can I import data from Quicken? - #6 by Patch
For how to do it see this post How to import data from other accounting software - #6 by Patch