'ただいま' literally means 'just now' or 'right now'. When you write it in hiragana, we would think you mean "I'm home", but if it is written with using kanji like 'ただ今' or '只今', it means 'just now' or 'right now' in general. '参上' means 'to visit someone' as you mentioned. You can use the expression when you come to see some superior. The word sounds a bit bookish and you seldom hear someone saying it in usual life, but you would hear it a lot if you love samurai dramas or something. 'ただいま参上' is a kind of a set phrase which is often used in period films. You would find that a ninja is saying it when he visits his master during you are watching a period film. He is saying 'I've just arrived here for you, my master'.