log in to the system as system administrator responsibility.  Hit CTRL+L at the initial navigation and this will bring up 'Form Functions' form.
Enter in '%flex' to narrow down the LOV.
Select 'Flexfield Value Sets'
 
 this is where things can get dicey.  The settings here depend
 on the type of parameters you have.  Are they standard dates? 
 Are they a list of POs or invoices or something along those
 lines?  I will tell you how to set up as if you wanted a list
 of values the user can select from and the list is based on a
 column of a table.

Enter in the value set name (something descriptive)
Enter in the maximum size of the value (longest value in LOV)
Enter in any other restrictions you may need (only CHAR, only number, ALL CAPS, etc.)
Under 'Value Validation' change from INDEPENDANT to TABLE.  This will enable the 'Edit Information' button
Under 'Table Application' select the module/schema the table is in
Under 'Table Name' select the table that houses the column your LoV is based on.  
Under 'value' select the column you want displayed in the LOV.
Set condiitions for the WHERE/ORDER BY clause if necessary.  If the column has non-unique values and you want to have unique values in your LOV you can use a WHERE EXISTS (SELECT DISTINCT column_name....) to get around the limitations of the Value Setting.
Save your changes
Exit back to the navigator
Go to Concurrent Program Definition
Query up your report
Click on 'Parameters'
Enter Seq # (I usually do these in increments of 10 if I have multiple parameters so first one is 10, second is 20, etc.)
Enter in parameter as it is defined in your RDF (P_PURCHASE_ORDERS or whatever it is)
Under 'Value Set' click the LOV button (...) and query up the value set you created
For prompt enter what the user would see in text to describe what they should enter (in this example maybe it would say 'Select a Purchase Order:'
Token is the parameter name again (should be the same as what you entered in 'parameter' - in this case P_PURCHASE_ORDERS)