There is no way to switch from R-O to R/W or visa versa from within PasswordSafe like in the windows version. Most of the time I open my password database in R-O mode so I or anyone else can't accidentally change something. To get around this for now I have two different shortcuts or launchers one with the command line "-r" option and one without. So to edit I close the R-O one then open the R/W one and then add or edit entries. I then close PassordSafe and then run the R-O launcher. Many steps.
Running the latest 0.92 Beta for 12.04 LTS amd64 uploaded 3 days ago on Mint 13 MATE