Hi @caslasgee,
Modify your formula as below:
Filter('Funding activities',UnitID=LookUp(Unit,Unit=dd_control.Selected.Unit).ID)
Note that I refer to dd_control.Selected.Unit, which depends on how you set the Items property of the dropdown control, if the formula you set to Items property returns Unit, then you need to refer to Unit, if it returs a column named Value, then you need to refer to dd_control.Selected.Value
To create new activities based on the Unit, I think you need to add a Form control, and set the following properties.
1). Add a button and set its OnSelect as:
NewForm(Form1)
2). Set the Item property of the Form1 as:
Gallery1.Selected //For editing an existing record from the gallery
3). Add a submit button and set its OnSelect as:
SubmitForm(Form1)
4). Remember to link the form to your activities list and add the fields.