Word processors, like Microsoft Word, and other computer programs that are intended for typing have a spell-check function. Because many older people don't want to admit that they don't know how to spell, a great way to help them is to have them learn on their own. Simply read sentences out loud for the older students, and have them type them in their Word documents. When they fail to spell a word correctly, that word will be underlined or highlighted, and they'll have to try again. Even if they right click and see how the word should be spelled, they will still be learning how to correct it.
Many low-level spelling lists have all kinds of words that people aren't going to use. When teaching older students, it is important for the words to be meaningful. Create spelling lists for older students by figuring out what is most important in their lives. For people who are trying to get jobs, words like "interested," "interview," "application," "tomorrow" and other job-search-related words are important. For older people who need to do other things, such as shopping or banking, other words ("checkout," "withdrawal," etc.) will be important. If you're a teacher, take a poll of your students and see which words are most important for them to learn. Then, create individualized word lists based on those words.
Many adults who would like to learn how to spell also have issues with reading. Using a daily or weekly newspaper as a starting point, take your reading and spelling lessons from there. When the words relate to real-world situations that older people want to learn about, they are going to be more likely to concentrate on learning how to spell those words.