There are a lot of different transmission lines in ADS. There are ideal lines, coaxial cable models, printed circuit board traces, coupled lines, etc. For an ideal line, the simplest is TLIN, which just has Z, and an electrical length that is described as a phase shift at a specified frequency. The 2-Terminal transmission line only has two places to connect a wire. The 2-Terminal model doesn't mention if there is a ground plane or a coaxial shield. The 4-Terminal ideal line adds extra connections for a ground plane or coax shield. For physically realistic models, you have to define a substrate, dielectric constants, trace widths, and so forth.
Another ideal transmission line is the TLIND element, which has a characteristic impedance and a delay in seconds.
After you place the TLIN or TLIND component, double click it to bring up the form. The Help button on the form takes you to the documentation.
The online documentation is at http://edadocs.software.keysight.com/display/support/Knowledge+Center
You need to be a registered user to get to the online manuals.
Find good materials available without registering on Youtube. There are a lot of ADS videos there, with many created by Keysight. Perhaps "Getting Started with ADS" would be helpful.
Full disclosure: I don't speak for Keysight Technologies, even though I work there. I don't think anyone from Keysight regularly visits this StackExchange site, and this is usually not a great source of ADS help.