Dehumidifier with multiple float switches
Your instructions are basically telling you this:


Remove the JUMPER from Float Switch terminals. 
Wire the Float Switch to the terminals (you should have two wires - one wire goes to one terminal the other wire to the other terminal) Does not matter which wire just that they are wired one to one terminal and the other wire to the other terminal. 
The Switch makes the connection or breaks the connection.  Considering it is Jumpered you want the Switch to open when the overflow level is reached.


To Answer your question about series or parallel:


Both Switches Normally Closed in Series.
A AND B = ON 


A Closed / B Open = OFF ,
A Open / B Closed = OFF , 
A Closed / B Closed = ON , 
A Open/ B Open = OFF. 

Both Switches Normally Closed in Parallel.
A OR B = ON 


A Closed / B Open = ON,
A Open / B Closed = ON, 
A Closed / B Closed = ON , 
A Open/ B Open = OFF.