Ah, sorry... there's all sorts of things that can make it more complicated. Well, first, what format is your comic file in? A zip, rar, tar, tar.gz or tar.bz (bzipped and gzipped tar files) are archives that contain images (jpg, png, etc). Sometimes, cbz or cbr are used instead, but those are just zip files and rar files with a different extension so a comic reading program will open instead of an archive unpacking program. You can also just use a folder with a collection of images in it, but it's less convenient.

Once it's on the SD card (N810 or N900) you can open it by running Comix, tapping the "Comix" title part for the menu, and going "File" then "Open". Right now, with Comix, you probably start out in the /home/user directory your first time -- browse to the SD card by tapping the "Up" arrow in the top left until you no longer can, then go to "media" and "Memory card".

Once there, you should be able to select the comic file if it's one of those.

CBR Pager has the proper hildonized file open dialog, which should just have "N900" and "Memory Card" as options. Same basic directions, except no need to go to the "Media" folder -- and you probably start out at the fake 'root', too.