As you said, that's the literal meaning of the sentence. Actually, the sentence is a proverb. It means "When we have a good chance (accidently), we deal with the important thing first." In ancient times, 떡 was special food which couldn't get easily. 제사 was the most important rite in a year. When koreans had 제사, they had some difficulty in preparing food for 제사. Here 떡 stands for 'a precious chance,' 제사 refers to 'very important thing.'

There's a similar English proverb which I like, "Make hay while the sun shines". 해 본 김에 건초 만든다 (Literally, you make hay at a chance to see the sun) You make hay while the sun shines. 떡 본 김에 제사 지낸다 You hold the memorial service at a chance to see the rice cake. Hope this helps.

It is little hard to explain this, but I will do my best. First of all, '떡' is one of important things in the ancestral memorial rites. So, '떡' can naturally remind the ancestral memorial rites. But, not all people do the ancestral rites impulsively when it is not planned. '떡 본 김에 제사 지낸다' is an exaggerated expression for the people's unplanned impulsive behavior when they have a trigger. For example, let say a person is waiting for someone in a library and find a computer at the lobby. What is this person likely to do? Surfing the net! This situation can be expressed as '떡 본 김에 제사 지낸다'. Hope it helps.

rice cake is korean traditional food, especially in the ancestral rites. So, this sentence means when you meet a chance, don't lost the chance, do it immediately. :)

떡 본 김에 제사 지낸다: Since you`ve seen the ddeok (rice cake), you spend [your time at] the ancestral rites [where ddeok is usually present]. This is used when you`ve already done something and you figure that you might as well do something else related while you`re at it. =)