The USB guitar link is for recording your guitar into a computer. Useful, but not relevant to your needs here.
To play something like a CD player through a guitar amp, it is sometimes enough to connect the headphone output of the CD player, to the instrument input of the amp. You will need to turn the headphone volume right down, and turn down the gain/distortion on the amp - because the amp will distort a strong signal. Remember a guitar amp is designed to colour and distort the sound.
However, if what you want to do is practice alongside recorded backing music, you would probably be better off letting your guitar amp do what it's good at -- amplifying a guitar -- and have the backing music come from a separate amp and speaker. A cheap boombox or a set of PC speakers would be fine. This way you can adjust the tone of your guitar, without affecting the backing music.
If you plug your guitar into the guitar amp, connect the amplifier's line-out to the Tascam's line-in, and listen to the output of your Tascam through headphones (or some other amp/speaker setup -- one designed not to distort or colour) -- then you are effectively using your guitar amp as a bulky effects pedal.
That's fine, but note that some of the tonal qualities provided by the guitar amp come from the physical build of the speaker and cabinet, and of course if you're not using those, you don't get the benefit from them.