Tomboy requires Mono, which AFAIK RHEL does not ship. You might want to take a look at Gnote, which provides similar functionality but does not require Mono.
Gnote is available in the RepoForge (formerly RPMForge) third-party repository for Red Hat Enterprise Linux. First, enable that repository and then you can install Gnote with the following command:
yum install gnote
Unfortuntately, they don't build it for RHEL5, but you can rebuild an RPM using the source RPM (SRPM). To do that, first you need to download the SRPM. Then, tell YUM to install rpmbuild if you don't already have it, by running this as root:
yum install rpmbuild
Next, install all dependencies needed to build it, again as root:
yum-builddep gnote-0.7.2-0.1.rf.src.rpm
Now you can rebuild the package using this command, which you should not run as root:
rpmbuild --rebuild gnote-0.7.2-0.1.rf.src.rpm
That will build and drop an RPM in ~/rpmbuild/RPMS/<arch>, where <arch> is i386 or x86_64 depending on whether you have a 32-bit or 64-bit operating system installed. All you have to do then is install it, as root this time:
rpm -ivh gnote-0.7.2-0.1.el5.rf.i386.rpm