OK ive seen people are still struggling with .RAR tutorials so ive thrown together this for complete beginners.
An important note: WinRAR does NOT require you to buy it to continue to use it, it will nag you when you open it but purchase is not necessary.
OK here is an absolutely fool-proof .RAR tutorial.
The roms that you download from romUlation.net are contained inside a compressed archive, similar to a .zip file. These archive are usually in .rar or .7z format. To use your rom you first need to extract it from the archive.
To extract them, first you need a program:
If you run windows download from here:
http://www.rarlab.com/rar/wrar370.exeIf you run a Mac download here:
http://homepage.mac.com/krmathis/OK now that you have installed WinRAR right-click on your file that you have downloaded (this will likely be XXXX - GamesName (U)(BobbyJohn).rar)
Scroll down to 'Extract here'.

WinRAR will then extract your file/s
The next step is usually
not necessary, only a few ROMs downloaded through the forum requires this. Skip to the next image if you downloaded the ROM via the site.
If the extraction turns up a .nds file then this is what you want to send to your Flash Cart or open with an emulator.
If the extraction turns up another .RAR files or a series of .RAR files then DO NOT PANIC!!!!
Simply Right click the first .RAR file in the series (this will be XXXXXXX.R00 or similar).

Scroll down to 'Extract here' and WinRAR will extract the files and the result should be...
A .NDS FILE! (or other file if you downloaded something other than a NDS rom, e.g. .ISO,.MDF,.BIN, ect.)

Note there may be a txt or info file along with the ROM, in the form of a .nfo file, this contains release information and is not required to run the game. Just ignore it, or if curious about the contents of such a file, rename it to .txt and open it, or get a program designed to view NFOs such as DamnNFOViewer.
If you hit an error then try re-extracting the first .RAR file, if this fails then re-download the ROM as it may have failed to download properly.
If all of this fails check the forum to see if anyone else is having problems
If not try re-installing WinRAR.