"So, how can I use this?"
Syntax: /pastebin to open up the dialog.
I've tried to make it as user-friendly as possible, but maybe it's not for some of you. So, here's some elaboration!
As you can see in the screen-shot below, the dialog itself is pretty massive. Plus, there are no tabs.

"So, how does uploading and downloading work?"
For uploading, you can upload existing files or paste them directly into the large edit box. You can add your name, a custom sub-domain name (for private pasting), how you want PasteBin to read your paste (72 languages to choose from), when you want your paste to expire, and much more!
For downloading, you can do two things. Firstly, you can simply preview it. Previewing is simply looking at the paste; no saving to your files or anything. This can be useful because if you save every single time to view a paste, it can waste your hard disk space. Secondly, you can download it. All files are downloaded to a folder called "PasteBin" in your mIRC Directory. If the paste was private, separate folders inside the folder "PasteBin" will be created, along with a modification of a file name depending on the type of paste (language syntax highlighting). You can also check the "Load to remotes?", to...load the paste to your remotes.

"What other features are available?"
The dialog is pretty much this:

* An uploading section
* A downloading section
* An information box
* A very large edit box
* A few buttons all around


Here are some features:

* Choose own name for uploading
* Options of private pasting (custom sub-domain names)
* 72 language highlighting options
* Selection of post expiry
* Importing (browsing) existing files
* Option to load paste to remote
* Clearing the screen
* Copying selected text or all text to the clipboard
* "Go to Nth line..." option
* many more!


So, I guess it's pretty self-explanatory.

*** Note: If you haven't used PasteBin before, I suggest following that link and testing it out for yourself before you test this dialog.

One more thing I would like note:
Use mIRC Version 6.34+ or many parts will not work! Enhancements with storage of variables and many other updates have been made in the newer versions, so hence, users with older mIRC versions will experience problems!

Some people I would like to give credits to:

* Firstmate for giving me crucial advice on a "on sockwrite" event, which allows faster + safer pasting (especially on large pastes).
* Blitzjager for taking a nice screen-shot for me (since my theme is terrible).