Off hand, you have way, way to many table occurrences between your layout's table occurrence, Project and the table occurrence that you appear to have selected to format as a drop down list: Location. I am guessing that ::LocationNam refers to the LocationName field in the Location table as I cannot see any such field listed in any other table and the name of the field and your drop down would be consistent
Just the chain of 3 occurrences linking Project to Presentation could be problematic but if you can get presentation records to appear in your portal, that may work for you..
But there appears to be two basic problems here. In order for your field to work, the drop down list would need to at least modify a record in the location table or at worst, create a new record in the location table. That does not appear to be what you want to do here judging from your layout design.
From your layout design, it appears that you want to specify a location for a specific presentation record. That might work if you formatted the LocationID field in the PresentationLocation table so that you could create a new record in that join table lining a specific location record to your presentation record.
But that raises the second problem with your current design. You have set this up for a many to many relationship between presentation and location. This implies that the same presentation can be linked to multiple locations just as a given location record can be linked to multiple presentations. But your current layout design will not permit specifying multiple locations by creating multiple records in the PresentationLocation table even if your drop down list was working.