How to find the fitting version of pgadmin III?
The most up-to-date version is always available with the most-up-to date version of postgres. So the coming 1.14 of pgAdmin support 9.1 with it's features, 1.12 -> 9.0, 1.10 -> >8.4. I noticed that pgAdmin will disable (or disallow you to use) certain features that do not support your installed pg version.  

Sidenote: I made bad experiences with using the build-in backup/restore tools from pgAdmin (on Linux), no idea why this wasn't working but one machine just wasn't able to import a backup made by another one with same version, specs,... Had this issue many times, so I always prefer the command line export/import. The latter has never failed and offers more options for exporting and importing.