How do I create beam elements?
Exacltly the same way as all other elements. Click on the first node then the last node then click back on the first. The element will then turn green to show that the program has closed the element successfully. Define material properties for the beam by selecting BEAM in the list of material models. Then select the beam elements and assign them this property.

When do I use 2-noded beam elements as opposed to 3-noded elements?

If the element representing the beam is meant to be embedded between the two adjacent 2D elements, then it would have to be a 3-noded beam. This would insure that the mid-side node of the beam is the same node as the mid-side nodes of the two adjacent 2D elements as can be seen in the image below:

If the beam is to represent a strut (or tie) connecting two points, then a 2-noded beam would be appropriate. For example you might wish to fix a wall against a fixed point on the boundary in which case a single 2-noded beam would be sufficient. You must avoid using a 3-noded beam if it is not embedded in soil (or between two 2D elements) as the unrestrained mid-side node of the beam would create ill-conditioning in the stiffness causing the program to stop.

How do I fix a beam into a wall made of 2D elements?

If you wish to make your beam fully fixed to a wall created with 2D elements, then you must extend your beam so that it is embedded into the wall as shown:

Remember that connecting the prop to a node on the wall would create a pin-joint connection rather than a full fixity. This is because 2D elements do not have rotational degrees of freedom.

Amir