I don't have the G55 so I can't give you very specific help, but I'll try. Looking up kanji is tricky. When you don't know any of the pronunciations, you usually have to rely on stroke count and the radical.
Stroke count is relatively easy to figure out. 郵 has 11 strokes. 便 has 9 strokes. There might be a box on your kanji lookup screen labeled 総画数 where you would enter this number. The problem is that on my dictionary, that yields 575 results, so unless you want to weed through 575 kanji, you have to narrow it down a bit more. The main radical is a way to do this but it's pretty tricky and requires some guesswork. In 郵 the main radical is that part on the right and in 便 it's that part on the left. On my dictionary (and hopefully yours is similar), there's a box labeled 部首画数. In here I would enter 3 (the number of strokes in the main radical of 郵) and then I would be provided with a small list of 3-stroke radicals and would select the right one. So if I search using the radical by itself I get 27 matches, and if I search using the radical and the total stroke count there are 5 matches, including 郵. Either way, much more manageable than 575.
Let's say you've found the character in your dictionary, then what? Probably you'll see in the kanji's entry a part called 常読 or 字音. This would have the character's on-yomi in katakana, and the kun-yomi in hiragana. This is where the guesswork begins, trying to piece together the readings of the kanji you've looked up to make a word. In the dictionary, you'll probably find that 郵 can be read as ユウ or ユ and 便 can be read as ベン, ビン or たよ・り. The correct combination, of course, is ゆうびん. This can all get really frustrating, and the best thing you can until you've got a lot of kanji under your belt is to expose yourself to reading sources with furigana so you don't have to guess or look up the readings.
Lastly, though I don't own the G55, I have used it briefly and I was also frustrated that it didn't have words with suffixes (like 郵便局) as headings. I guess it's just something you've got to get used to. Good luck