Go to the posts you want to quote.
Bottom right hand side of post, there is a bubble with a + next to it.
Click on that symbol for each quote you want to include in your multi-quote.
On the last quote to be included in the multi-quote, click on 'Reply With Quote' as well as the symbol next to the + sign.
Just a tip, I then click on Go Advance so I can see better.
Also, MDS has a pact with satan and you can only have three images per post. If there are more than three images, you will get an alert. You will either have to delete the emoticon codes, or not quote as many posts. I usually delete some of the emoticon codes in the posts I am quoting, but it sucks if you want to use more than three. Sometimes, I just describe what emoticon I want, like; :putasmileyfacehere:
I hope this helps!