Assets are parts of your system that are of value to at least one of the involved stakeholders. More formal: if the loss of any of its security attributes leads to the violation of any of the stakeholders' objectives, it can be treated as an asset. Asset Identification is merely the process to determine if a candidate asset is truly an asset. All parts of your system design in medini are based on SysML and as such by default "candidate assets". During the Asset identification process, you can mark them truly as an "Asset" and add security attributes.
Select an arbitrary element of your design, either in the model browser or on any diagram. You can select structure elements but also behavior elements.
Open the Properties view and switch to the Security tab. To mark an element as an Asset, simply check the "Asset" check-box in the tab. You can uncheck it at any time later. Note that making an element an Asset is a prerequisite to add security attributes.