Your Library folder is there, just hidden. Apple decided too many people were messing with stuff in the Library folder breaking perfectly good apps and then blaming Mac OS X, so they hide it.
Finder -> Go -> HOLD THE Option (Alt) Key -> Library
That is to say the Finder Go menu while holding the "Option" key will then display "Library" folder that can be opened via the Finder Go menu.
Or Finder -> Go -> Go to Folder... -> ~/Library (where ~ is the tilda key)
What is Mail.app reporting for errors or for diagnostics, or what are some of the unexpected behaviors you're encountering, when it "doesn't work"?
If you can get to this particular tool within Mail.app, what does the Connection Doctor report? (Mail.app > Window > Connection Doctor)
As for your question, there are two library folders around for each user. The first is the system-wide Library folder. The second is a per-user Library folder. Both folders are around and both are involved with Mail.app, though they're not visible (by default) in recent releases.
It's comparatively rare for a user to have to directly access either of these two Library folders — and changes made to these folders can cause problems, which is why Apple has hidden them.
What "patch" did you download, and from what site did you download it?
There are many Mac-related web sites, and Apple has many tools posted. Please post the specific URL. (If you're not sure of the URL, then you can Control-Click on the file that was download in Finder, and select Get Info from the pop-up menu, and that display can sometimes provide you with the source URL for the file.)
As for opening specific mail messages causing issues for Mail.app, that can imply a problem with those specific mail messages, or potentially a mailbox corruption.
First, select and issue a mailbox rebuild <Mail.app > Mailbox > Rebuild) for each mailbox involved with the problem message(s). This can rebuild each mailbox, and can sometimes resolve these cases.
If that rebuild fails to resolve this, then one of the easiest ways to resolve that is to log into the web interface from your mail provider (if one is available), and delete the specific messages invoilved. (How you log into the web mail interface (and whether or not your mail provider offers a web mail interface) is specific to the mail provider.)