for example if you have two lines, with a gap between, on a substrat (the substrat and the lines are create by 'Draw->Box', for details see help in HFSS). then you make 'Draw->rectangular', which creates a 2D sheet. The sheet has to fill the gap between the lines. After that you can click on the sheet and select 'HFSS->Boundaries-> Assign->Lumped RLC'. For the Current line open the list on the right (item selected by default: 'undefined' ) and choose 'New Line'. Then select the two connection points of the element in the drawing window on the sheet. You will see a red arrow on the sheet.
For the first steps in hfss (making objects and so on) search for tutorials in the web, there are some like making a dipole. For a lumped port (e.g. lumped RLC) you can look at : **broken link removed**