When adding a Write Up it will automatically pull through the Primary Listing.  If you have multiple Listings on your Lead you can select from these Listing by clicking on the green 'SELECT' button.  Any 'Other Listings' you have on your Lead will able to be selected.  If you wish to select another Listing that is not on your Lead you can use the 'Select more' to browse your entire inventory.  Any listing you select here will be back-filled as one of the 'Other Listings' on your Lead record.

If you wish to add multiple Listings to your Lead you can select 'ADD' under the 'Other Listings' section.  This will enable you to choose from any of the Other Listings you have on your Lead.  You must have the Multiple Listings Write Up role enabled.  When you add multiple Listings to your Write Up Form it will sum these when calculating the totals in the Write Up Form.  Note - make sure to check whether your Write Up Form Template is compatible with displaying multiple listings.  If multiple listings are not showing up on your form contact Support to request a change to your template (additional charges may apply).