I'm guessing you've installed new OXCE to a different folder other than your old installation, and thus you can't access them from your new OXCE, that expects them to be in a different folder.
So you need to locate your old installation and copy the saves from there to your new OXCE. They usually live at ...OXCE/user/x-com-files, or sometimes in user documents.
If you have no idea where old OXCE is, do a drive-wide search for "x-com-files" and/or "*.sav" files, which should help locate the saves in general, or search for "OpenXCom.exe" for old installation.
Also russian VK x-com-files community may be better equipped to help with this.