There's a separate 'manual' on the Easier to Read' site for the 436HP/536HP version of Sentinel.
BCDX36HP Sentinel Software Manual
If you have not already done so, I would advise against clearing the already programmed memory card until after you saved it's comments to your PCs hard drive. Or, keep that card as a semi-spare, and start fresh with another new card. With the database scanners, both Uniden & Whistler (as well as the ones manufactured by GRE), it's always a good idea to have a spare card handy,
In Sentinel, you can add additional systems by appending them from the main database. You can remove a system, or only parts of it that are of no interest (or are out of range) either by using the Favorites list editor, or by right-clicking the item to be removed in the opening Sentinel screen, then selecting Remove from the pop-up menu. If removing portions of a trunked system, like a series of talkgroup IDs. be sure not to accidentally remove the site you need to monitor. Using Sentinel's Favorites list editor, you can add or modify channels (conventional systems) or talkgroups (for trunked systems) without having to completely start over.
If you can be a little more specific as to what's giving you a problem, then some would be able to provide more useful suggestions.
While you'll always need Sentinel for firmware updates, as well as database updates, there are two third party (paid) software packages that work with the 536HP, ProScan and ARC536. Of the two, I prefer
ProScan, but both have a 30 day free trial period so that you can test drive them before having to purchase a license.