If you have a regular asset (i.e., an asset not held in a tax deferred qualified retirement plan) with an unrealized capital gain, you have two options.
The first is to include this asset in your regular asset account and to specify under Assumptions and Settings > Taxes the amount of the unrealized long term capital gain in your regular asset account. In this case, MaxiFi will assume you will realize the gain and pay capital gains taxes on it smoothly through time. See here for details: Capital Gains and Qualified Dividends
Otherwise, you can realize the gain on this asset in the current or some f year by excluding this asset from your regular asset account and treat the sale of your appreciated assets as two special receipts. You'd enter the sale of the asset in year X as the combination of a) a non-tax related special receipt in year X equal to the asset’s projected basis in year X and b) a special receipt equal to the unrealized capital gains, which is taxable at capital gains tax rates.